Each fall the small town of Churchill, Canada, becomes polar bear central. Usually these animals travel alone, but in autumn they congregate here on the western rim of Hudson Bay, waiting for the water to freeze over. Once the ice forms, they wait above a seal's breathing hole, ready to snatch their prey. Early on a cold morning you head out to the bear fields in a tundra buggy, a white, school-bus-like affair. You live for several days in a huge mobile lodge, watching from a safe observation platform as these ungainly creatures cavort in the snow. Both male and female adults can be seen and sometimes, cubs. The males, some speculate, get to know one another here, assessing strengths and weaknesses and establishing rank before they head out onto the ice. You're likely to encounter other animals as well, such as the Arctic fox, the Arctic hare, and the snowy owl. TravelWild Expeditions. October, 9 days, $3,195 (includes airfare from Winnipeg, Manitoba)

There are lots of ways to see the Everglades, including at least one that lets you work to save its endangered wildlife. Several years ago researchers began releasing young captive-reared whooping cranes into the wetland, hoping to establish breeding colonies. But 60 percent of the birds released in the first five years were killed by bobcats. If the project is to succeed, the odds in favor of the birds will have to get a lot better.

That's where you come in. On this project you work with researchers from the Florida Game and Fresh Water Fish Commission a nd the U.S. Geological Survey's Patuxent Wildlife Research Center, in Laurel, Maryland, observing and recording the birds' behavior and helping new populations of this endangered species to survive. It's not glamorous-participants share a rental house and cooking duties-but you can consider it a working vacation. Earthwatch Institute. January-March, 14 days, $1,495

Yellowstone National Park is nearly empty in the winter-of people, that is. Snow hushes the landscape, and the cold draws wild animals down from the high peaks. Bald eagles soar high, and the tracks of elk, bison, deer, and wolves crisscross the park. The Bear Tooth, Absaroka, and Gallatin ranges, snow-clad, provide a dramatic backdrop for an afternoon of cross-country skiing. This trip shows you Yellowstone's winter wonder. Guided by carnivore ecologist Jim Halfpenny, you track and observe the animals that roam this rich ecosystem, including the recently reintroduced gray wolves. On other days Halfpenny and geologist Charles Woodward lead you on hikes into the Teton Range to study its geology. There are also visits to the Old Faithful and Lone Star geysers-not to mention a soak in the hot-spring pool at Chico Hot Springs Lodge. Founded 14 years ago by Janet Ross, the Four Corners School set out to "use the Colorado Plateau as an outdoor classroom," educating people about the need to preserve the Southwest. Today, in addition to the trips it sponsors, the organization runs archeological studies and an environmental-education program for local elementary-school children. Four Corners School of Outdoor Education. February, 8 days, $1,820

High above one of the Virgin Islands' prettiest beaches, there's a resort called Harmony, where the walls are made of recycled newspapers and the shower tiles were once lightbulbs. Solar panels store sunlight, and cisterns collect rainwater. On the roofs, wind scoops face the trade winds, funneling cool air into the rooms and doing away with the need for air conditioning. This is the future-and it works. The location is St. John Island. The resort is part of Maho Bay Camps, which lies at the edge of the Virgin Islands National Park where you can hike on forested hills or scuba dive or snorkel offshore. The resort itself offers sailing, windsurfing, and kayaking, as well as accommodations that range from tent cottages to studios-and a central open-air pavilion where you can listen to talks by the rangers at the park. It's a great place to hunker down with the kids at a price that's hard to beat. Maho Bay Camps. From $105 per night ($60 per night before December 14)
or

Santa Rosa Island, off Santa Barbara, California, is 54,000 uninhabited acres of grasslands, deep canyons, rolling hills, and marine terraces. Native mammals such as island foxes, deer mice, and spotted skunks roam here, and more than 195 bird species soar above the oaks and ironwoods. This second-largest of the Channel Islands, however, is not without flaws: Sheep and cattle ranching have etched scars across its landscape. Trip leader Kathryn Hannay, who has organized work projects for the past 10 years, invites you to help restore the island. The projects include beach cleanup, oak revegetation, and trail repair. In your free time-and there is some-you can hike the hills, snorkel the surrounding waters, and see some of the more than 160 archeological sites left by the Chumash Indians, who inhabited this island more than 8,000 years ago. Sierra Club Outings. October, 7 days, $490

No golf courses. No resorts. No hula dancers. Instead, a naturalist well versed in the Hawaiian Islands, who shows you the most magnificent sites on three of them, including pristine rainforests, the world's most active volcano, and beaches the other tourists never find.

On Kauai you hike the Na Pali Coast, with its emerald cliffs that plunge thousands of feet into the sea. Then fly to Maui, where footpaths run past salt-sprayed coastlines, tidepools, and rock formations sticking straight into the sky. On the Big Island you visit an amphitheater valley that was carved by wind and water some 800,000 years ago. You hike by day and stay at bed-and-breakfasts at night. Journeys International. October-March. 10 days, $1,700 (includes transportation between the islands)

The drive from Mexico City takes you through cool pine forests to the Rosario and Chincua butterfly sanctuaries. Here, monarchs swarm: black, yellow, and orange, covering the trees in moving flashes of color. Watching them, you begin to understand why the ancient Aztecs believed that butterflies were the reincarnated souls of the dead.

After two days you move down the coast to Puerto Escondido to explore Mexico's last stretch of virgin coastline. Lagoons and mangrove islands shelter alligators, black orchids, and more than 300 species of birds, among them bare-throated tiger herons, white-fronted parrots, and northern jacanas. Spend a day at the Marine Turtle Research Center and two in Oaxaca, visiting Zapotec ruins and stocking up in the crafts markets. Natural Habitat Adventures. January-March, 12 days, $3,195

Dominica is what the Caribbean might have looked like a century ago. Dense forests shelter African tulip trees flamboyant with scarlet blossoms; in their shadow live mosses and air plants, lichens and orchids. Mangrove swamps line the coasts, where herons, egrets, and pelicans roost. One species of parrot is found only on this island: the endangered sisserou. Offshore, brilliant parrotfish, colorful wrasses, blue-tang surgeonfish, damselfish, and angelfish weave through the coral. This trip takes you hiking through the jungles, snorkeling offshore, and whale watching. Nature Expeditions International. December-February, 8 days, $1,790

If ever there was a place to run away to, Esp íritu Santo Island is it. The turquoise waters of the Sea of Cort és lap at this uninhabited island off Baja California, in Mexico.

This is a haven for marine life, where tropical fish swarm, manta rays skim the water, and sea lions cavort. A naturalist guide teaches beginners to kayak, and then you're off: paddling the coast, heading into the canyons for a close look at the cacti that cover the island, or just watching the myriad dolphins and blue and finback whales that winter here. When you're tired of all that, you can put in to a sheltered cove and pitch camp on a deserted beach. Mountain Travel-Sobek. October-March, 7 days, $1,195-$1,295

Feeling adventurous? If so, the best way to appreciate Costa Rica's varied geography is to hike through it in the company of a knowledgeable local guide. In the course of a day you climb 7,200 feet through five climate zones and tropical rainforest to oak and cedar forests. Then you loop around the top of Cerro Chirripó-at about 12,500 feet, Costa Rica's highest peak. It's a rare trip, even for the locals. The reward is even rarer: a view that on a clear day takes in both the Atlantic and Pacific oceans shimmering in the far distance.

Besides the climb, there is whitewater rafting on the Class III and IV Río Pacuare, which runs through a rainforest. And you get to meet the people of Costa Rica, staying overnight in the home of a local guide and his family. Above the Clouds Trekking. December-March, 11 days, $1,625-$1,975

In the Darién jungle of Panama, you get around the way the locals do: on foot or on the water. This naturalist-guided expedition wends its way by foot and by canoe from the mangrove-forested Gulf of San Miguel down the Tuira River to Darién National Park, with its waterfalls and clear pools. Hiking along the Boca de Cupe trail to the lodge at Cana, you may be followed by macaws and red-throated caracaras. There are stops throughout the trip-at a ghost town (once home to miners), at an Indian village, and on the way back to Panama City, at the canal itself. The company that runs the trip is owned by Panama's National Association for the Conservation of Nature (ANCON); a portion of the tour company's revenue flows directly to conservation projects in the country. ANCON Expeditions of Panama. January, 14 days, $2,400

On a balmy night on the coast of Suriname-a country about the size of Oklahoma that's wedged tightly between Brazil and French Guiana-leatherback turtles crawl ashore to nest, a ritual that has been repeated for thousands of years. Leatherback turtle populations are declining, however, and they need your help. Under the direction of Oceanic Society sea turtle biologists and local forest service wardens, you locate the turtles as they crawl up the beach, move the eggs to areas where they can safely hatch, and later, chaperone hatchlings back to the sea. Oceanic Society Expeditions. March, 9 days, $1,895 (includes airfare from Miami)

Fishing on this trip may mean standing chest-deep in a lake in the heart of the Peruvian Amazon, seining for tropical fish as flocks of parakeets and blue and gold macaws call overhead. For most of the week you cruise the Río Napo, a little-visited tributary of the great river, in the company of two naturalists, collecting and studying stingrays, electric eels, river puffers, and piranhas. According to David Schleser, a former curator of the Dallas Aquarium who often accompanies the trip, piranhas have been given a bum rap. They don't often attack humans, preferring instead their usual prey, small fish and the occasional bird. Some of your catch-carefully monitored to sort out endangered species and return them to the water-goes home with you, and the tour provides locals with income. Other highlights: stops at the Atun Cocha lake for a look at the Victoria Regia water lilies, which can grow as big as three feet in diameter, and an overnight stay on a riverboat. Margarita Tours. January, 7 days, $1,773 (includes airfare from Miami)

The shrieks of wedge-capped capuchin monkeys overhead stop you in your tracks. You watch with fascination as the monkeys compete aggressively for small yellow fruits, but you also have a critical job: to record who eats first and to collect samples of fruits and leaves. You work at Hato Piñero, a 200,000-acre ranch in the llanos (or plains) of Venezuela, helping anthropologist Lynne E. Miller study the foraging activities and social behavior of the capuchins during the dry season. Immersed in a landscape of open grasslands and dense tropical forests-teeming with tree frogs, anteaters, and 300 species of birds-volunteers learn to appreciate Venezuela's fragile habitat and help expand local conservation efforts. Earthwatch Institute. January-March, 13 days, $1,595

High overhead a condor soars. Herds of guanacos troop by, a parade of fawn-colored creatures that look for all the world like small, humpless camels. The southern tip of Chile is a place of jagged peaks and sparkling seas. The trip begins aboard a ship cruising the narrow fjords. Later, you stay at a thermal resort, then fly to Punta Arenas to hike through Torres del Paine national park. Finally, you fly to Punta Tombo to see the Magellanic penguin colony in the company of Carol Mackie de Passera, who spent three years here researching her book Fight for Life. Geographic Expeditions. March, 18 days, $4,895-$5,575

There are dozens of eco-trips to the Galápagos Islands, but this cruise is run by a company that has been active in helping preserve wildlife for more than 30 years, supporting both the Galápagos National Park and the famous Charles Darwin research facility on Santa Cruz Island (one of the stops on the tour). The cruise-led by naturalists who have studied these islands in detail-also stops at Hood Island, where nocturnal green turtles bury their eggs to be incubated by the sun. On Floreana Island, flamingos feed in a crystalline lagoon, and on Isabela-the largest island-there are penguins.

Fernandina is home to marine iguanas and flightless cormorants; James is known for fur seals. You explore the islands on foot, reaching them by Zodiac rafts, which take you into narrow and hard-to-reach waters. A glass-bottom boat provides a window on the undersea world. Linblad Special Expeditions. October-February, 10 days, $3,770-5,460 (includes airfare from Miami)

Cruising the waters off Antarctica, you can see what drew generations of explorers to the white continent: icebergs as long as a city block, crystalline mountains that rear up suddenly from the sea, a flat polar plateau that seems to stretch as far as infinity. There are thousands of penguins-gentoo, chinstrap, macaroni, Adélie, rockhopper, Magellanic, and king. Elephant, leopard, and crabeater seals live in these waters; so do wandering albatrosses and killer whales.

The best way to see the continent is on a cruise, and this is one of the cushiest. You set out from Santiago, Chile, then stop at research stations run by U.S., German, and Russian scientists studying weather patterns, icebergs, volcanoes, and penguin behavior. You can also attend shipboard talks by guest naturalists. After a day spent exploring the ice fields by Zodiac raft, you return to the luxurious comfort of the World Discoverer-a specially equipped polar ship complete with a library, lounges, a cinema, and an exercise room. Society Expeditions. November-March, 13-20 days, $6,090-$16,550

If you really, really want to do Africa right (and don't mind selling your Dell stock to pay for it), sign on to this monthlong tour via train and private jet with expert guides such as zoologist Farish Jenkins, of Harvard University. Near Botswana's Chobe River you'll see huge herds of elephants. From the deck of a riverboat, you search out hippos and waterbucks. Then you travel through South Africa's wine country to the Cape of Good Hope, where two oceans-and two marine ecosystems-meet. From there it's an easy flight to Namibia's Walvis Bay, to see flocks of flamingos hundreds strong, then on up to Mali's ancient city of Timbuktu. National Audubon Society Nature Odysseys. February, 25 days, $27,950

Abercrombie & Kent was one of the first ecotour companies to run the truly civilized safari, with porters to set up camp each night and waiters in black tie to serve dinner beneath the stars. Nearly 36 years later, the tradition endures.

By day, guests witness the Great Migration: 1.5 million wildebeests and 1 million zebras and gazelles crossing the Serengeti Plain of Tanzania. The 14-day safari also includes camping on the rim of Ngorongoro Crater (an ancient volcanic caldera) and visits to the Tarangire and Lake Manyara national parks, where you watch elephants and black rhinos. Abercrombie & Kent. October-March, 14 days, $6,035

Called the Galápagos of the East, the Seychelles Islands, in the Indian Ocean, have been inhabited for only the past 200 years. In humanity's absence, nature ran wild: Tens of thousands of giant tortoises still lumber along the beaches, and a palm forest shelters coco de mer trees (found only here) and rare black parrots. You can snorkel or dive in the waters where Jacques Cousteau filmed his masterpiece, The Silent World, or go fishing or windsurfing. Or you can just relax-in a hotel where Noel Coward and Ian Fleming once stayed. Tamu Safaris November-March, 15 days, $2,995

This is the only tour to the Northeast Frontier Area of India, opened to foreigners just three years ago, and it includes a unique chance to stay at Kaziranga National Park. It's the only one in Asia that has all the big game: tigers, rhinos, elephants, buffalo, and bears. Naturalist guides accompany you on hikes along the rainforested peaks of Arunchal Pradesh, and you have a chance to meet some of the local people-representatives of the 21 tribes that live in these valleys, many rarely visited by Westerners. Geographic Expeditions. December or February, 17 days, from $3,045

Deep in the forests of Nepal, there is a sudden movement and a flash of startling color: dull orange against black. The tiger in flight is a blur of stripes, a profile in power. Once, these animals were stalked by maharajas and kings. Today they face more mundane-if efficient-threats: poachers and the loss of habitat. You travel through Nepal's Tiger Tops preserve and India's Bandhavgarh National Park by elephant, Land Rover, and foot, accompanied by naturalists from both reserves. This nonprofit trip has been given four times already, generating $40,000 to help protect endangered species. Mountain Travel-Sobek. March, 17 days, $3,350

In the shadow of the snow-capped peak of Mount Kilimanjaro, an elephant appears, ponderously making its way through the brush. It trumpets suddenly, the primordial sound breaking the quiet of a late afternoon in a remote reach of Kenya's Amboseli National Park. The elephant is there thanks in part to Iain Douglas-Hamilton, leader of this trip, one of Africa's foremost elephant experts, and the first to alert conservationists to the poaching problems that have decimated so many of the African herds. Expect to see hippos, buffalos, zebras, giraffes, and cheetahs as well. Discovery Channel Adventures. February-March, 14 days, $6,195

Sunlight filters through the clear waters off the tiny Micronesian country of Palau, highlighting wrasses and clownfish, sea anemones and nudibranchs. The most amazing creature of all is the giant clam, which can grow four feet across and weigh as much as 200 pounds. Two snorkel trips a day explore these shallow, calm waters, home to 1,500 species of fish and 700 kinds of coral. There's lots to see on land, too: The Rock Islands are floating gardens rooted in limestone. Oceanic Society Expeditions. February, 10 days, $2,590

If you want to see everything wild in Australia, sign up for this 21-day excursion by plane, train, car, and foot. You cross the continent with a stop for birdwatching in Sydney's Broken Bay and a sojourn in the outback, with its eerily glowing Ayers Rock. After a flight to Tasmania, you stay on Cradle Mountain, where you can hike the forested valleys and waterfalls and lakes sculpted during the last ice age. There are wombats and wallabies, as well as the fearsome Tasmanian devil. Next you fly to Cairns, to hike in the Atherton Tablelands, searching out tree kangaroos, bowerbirds, and butterflies. Cairns is also the gateway to the Great Barrier Reef. Here you can snorkel or dive-or just watch the water through the glass bottom of a cruise boat. Lindblad Special Expeditions. September and October, 21 days, $6,990-$7,290

The companies mentioned in "The Great Eco-Trips" are listed below, as are several others that specialize in birdwatching (indicated by a ). Many more companies offer eco-trips. For a list, call the EcoTourism Society at 802-447-2121. The society also lists outfitters and suggests questions to ask of them on its web site (www.ecotourism.org). The "Green" Hotels Association offers a list of hotels that take measures such as reducing waste or saving water. Call 713-789-8889 or visit the group's web site (www.green-hotels.com).