DRIFTING SILENTLY ALONG THE LAKE Janauaca shoreline, cloaked by the blackest of Amazon nights, ignoring the shrill warbling of lovesick frogs, I focused my attention on the disk of light we were casting onto the nearby shore. I hoped to see eyes in the water there. Caiman eyes.

Lake Janauaca -- part of a system of lakes draining into the River Solimoes, about 20 miles west of the port town of Manaus, Brazil -- is home to shoals of piranhas, a few huge water snakes and countless spectacled caiman. Millions of this small member of the crocodile family thrive in the watery vastness of Brazil's Amazon Basin.

By chartered riverboat, our floating home for a week, we had breasted the great Amazon River for a day before leaving it as one would exit a freeway to wend the basin's watery byways. Slow, careful navigation brought us to the solitude of tree-girt lakes alive with colorful wildlife.

In the light of day, Amazonia's water creatures rest and hide from the sun. But with the final crepuscular rays, they emerge to feed. Caiman, for example, like to drift slowly in the darkened water in search of frogs and fish. Now, we drifted slowly in a canoe in search of caiman.

The previous day, as we explored an igarape (literally, canoe path, or creek) in our 30-foot canoe, a young caboclo had approached us in his tiny dugout canoe. (Caboclos are the descendants of Spanish or Portuguese settlers who mated with indigenous Indians.) The cinnamon-skinned boy offered for sale freshly caught "bodo" armored catfish, a species that digs holes into the riverbank below the surface as a repository for its eggs. When we declined to buy, he brought forth a three-foot spectacled caiman. Trussed up in a harness of string, the creature lay motionless across the boy's bare legs. The retinas of its exophthalmic periscope eyes were closed to slits. It projected a patient, hopeless indifference to being lifted up and handed around. The boy offered to let us take its picture for a price, but refused American money. We had no cruzeiros, so he paddled away with a shake of his head.

That afternoon, we reboarded our ship, the Alyson -- a 65-foot riverboat handcrafted by a shipwright at the village of Novo Airao -- to cross the mile-wide Solimoes and enter the Janauaca chain of lakes. Pink river dolphins rolled in the green water, pursuing red piranhas and peacock bass. Squirrel monkeys, the kind that once collected coins for organ grinders, flung themselves through the branches of a towering kapok tree, chattering and squeaking. A flock of primitive hoatzin birds clambered among the branches of an acacia, preferring the tree's security to flight. About the size of a large pheasant and sporting a turkey's square tail feathers, the hoatzin wears too much blue eye shadow and a blond fright wig, giving it the look of a terrified feather duster. The birds hid themselves behind leafy branches, gobbling and squawking their alarm calls. Caboclos call them "stink birds."

At anchor in the quiet lake, we dined on deck. A dinner of broiled fillets of an extravagantly colored macaw catfish purchased from a caboclo fishermen earlier in the day and a dessert of guava jelly and clotted cream restored our sense of adventure. We resolved to find a caiman of our own.

The Alyson's deck hands clamped an outboard motor onto the transom of the canoe, and we droned off into the soft darkness of the warm, humid night.

During the day, a spectacled caiman likes to bask in the sun beneath overhanging brush at the water's edge. As long as your boat keeps moving, the dozing reptile remains motionless. But still the engine or turn in its direction and the caiman vanishes instantly in a swirl of muddy water. At night, though, when blinded by a spotlight's beam, the lizard remains transfixed and can be approached.

The water here, 3 degrees south of the equator, was fairly clear, its burden of Andean silt precipitated onto the flooded earth. Our light penetrated to a bottom that reflected colors of amber sand and green moss. The silver undersides of big, floppy cecropia leaves gleamed like mirrors and cascading yellow blossoms of a marimari tree stood out sharply against lush green leaves.

Two reddish dots appeared in our window of light. Caiman eyes. We let the canoe drift silently toward them until we could make out the olive green scutes and scales on the caiman's back and sides. It was a youngster, about 30 inches long, just half grown.

I had done this as a boy in East Texas river bottom lakes. Fifty years ago, spotlighting alligators at night, we shot them between those glaring eyes with a .22 rifle. Now, in an Amazonian flood plain, the same basic method still worked without the gun. Holding the light steadily onto the reptile's eyes, I stepped into the warm water. With one quick grab, I seized the dazzled lizard just behind its head and snatched it out of the water. Its tail flapped twice, then hung motionless.

The caiman's wet skin, though slick, was not slimy. I stroked the smooth, yellow-white plates of its stomach and examined its four-toed front feet. The long narrow jaws, partly open, exposed some 50 round, pointed teeth.

For just a moment, I held my basilisk, memorizing its wet plastic feel, sensing its fearful tenseness, defying the mythical venom of its reptilian eyes. Then I placed it back in the water and opened my hand. It floated there for a few seconds, perhaps assessing its freedom, then lunged away with a powerful thrust of its muscular tail.

Following a morning conference over poached eggs enlivened with the skipper's homemade pepper sauce, we decided to take the Alyson back to the Solimoes and motor upstream to the town of Manacapuru. There we would turn south again into Lake Cabaliana to photograph the birds convening near the lake shores. In Manacapuru, we hired a guide, a commercial fisherman named Julho de Souza Coelho, who could see birds where we could not. His interesting name translates as July Rabbit.

Far up Lake Cabaliana, Julho steered the canoe toward colorful wattled jacanas feeding like Rhode Island red chickens in the patches of floating grass. But the wary birds dashed away on our approach. Cocoi, a large white-necked cousin to the great blue heron, likewise wouldn't tolerate closeness, and huge flocks of Muscovy ducks flushed into the substratosphere on hearing the sound of our outboard motor drawing near. Taking pictures with a hand-held camera was an exercise in futility.

Exasperated, I asked Julho to put the boat onto dry ground where I might hide with a tripod that would enable me to use longer lenses. With powerful paddle strokes, he forced the heavy craft through meadows of floating grass onto shore. There I was able to step into dim green shade beneath a dense upper canopy of leaves.

Almost immediately, I discovered why this was not a good idea. Ants seemed to materialize out of the ground to clamber in purposeful platoons up my legs and those of the camera tripod. Mosquitoes and small stinging wasps dive-bombed my bare arms with high thin cries of delight. Even Julho, inured to the jungle, began to look pained.

In Brazil, there are millions of species of insects, more in number than all other living things put together. Lacking the steel-wool fur coat that protects the three-toed tree sloths from their formic aggressions, Julho and I retreated to the boat and spent the next 20 minutes removing our unwanted guests.

Back on the Alyson, I took a long soapy shower with special attention paid to the creases and crannies where red bugs and ticks might lodge, and decided that using the Alyson itself as a floating photography platform might be a better choice. So I asked Carlos to throttle the vessel's diesel down to dead slow and cruise along the shoreline.

When hawk-eyed Julho saw an interesting bird, the skipper would stop the engine and steer the boat into shore. Most birds, used to constant boat traffic, would pay no attention to us. We could take their pictures from the deck with tripod-mounted cameras and long lenses while enjoying relative safety from the insect hordes, shade from the deck awning and close proximity to the beer cooler.

Wary horned screamers, for example, sat like large black muffins in the tops of tall trees and yoo-hooed to their relatives on the other side of the channel. A large-bodied bird the size of a turkey, the screamer has a ridiculous little head from which sprouts a single black plume bearing little resemblance to a horn. Farther downstream, a black-capped donacobius sat calmly on a cecropia limb and showed off his yellow vest, orange westcoat and golden eye, all framed in black. Long-billed Amazon kingfishers wearing Miami Dolphin uniforms of tangerine and turquoise perched on branches overlooking the water where they watched for unwary fish.

Now and then, a boatload of schoolchildren or fishermen or town-bound shoppers would pass. Otherwise we had the river to ourselves. No jostling crowds. No lecturing guides. No schedules to keep.

Electing to sample a different riverine ecology, we left the silty "white water" of the River Solimoes, which drains the Andes watershed, and sought out the clear "black water" of the River Negro, which drains the rain forest northwest of Manaus. Stained with tannin like the water in the bayous of the American South, River Negro water has a pH so acid that the larvae of mosquitoes and other insects cannot survive in it. That alone is a powerful recommendation for adventuring up the Negro. On the other hand, the lack of nutrients in the water and the absence of insects means a smaller prey base for wildlife. The great numbers of birds, fish and mammals common along the Solimoes are less in evidence in the Negro watershed. People too are fewer in number, so that wildlife there is less pursued and more trusting.

In three hours, riding the swift current, the Alyson arrived at the juncture of the Solimoes and the Negro, where the famed wedding of the waters occurs and the Amazon, second largest river in the world, is formed. When Spanish explorer Francisco de Orellana came here on June 3, 1542, he found the Manao Indians "a people of medium stature, of very highly developed manners," who deeply resented his presence. He was glad to get out alive.

It's different now. About a million people of medium stature and highly developed manners populate modern Manaus, but tourists are welcome. A 17th-century city that boomed in the 19th century on the profits of the natural rubber trade, Manaus preserves its past in its splendidly rococo opera house -- where Caruso once sang -- its British Customs House and its fish market, all patterned after European originals.

More recently, the city's population has been swollen by young caboclos seeking a life less dependent on the jungle's dwindling natural resources. Overfishing, for example, is steadily depleting the Amazon's fish population. Julho told us that 20 years ago, he regularly caught pirarucu weighing more than 300 pounds. A large-scaled fish with a bony tongue, the pirarucu is said to be the world's largest freshwater fish; it can reach a weight of 500 pounds. Julho also caught piraiba up to 400 pounds. These giant catfish were so large, he said, that they had mouths 18 inches wide and could swallow a man.

Not only do such monsters no longer exist, but the numbers of all species are down. A local favorite, the tambaqui, is listed as a threatened species, and its market price has risen so high that "only the rich can afford it," he said. Too many fishermen chase after too few fish, and Julho can scarcely support his wife and two sons with his catch. For his boys, Julho wants a different future. "I want them to get an education and make a living with their brains instead of their hands," he said, "se Dios quize." If God wishes.

Pushing upstream against the River Negro's dark flood, we turned north into the River Cueiras, the River of the Calabash Trees, where farms were few and trees met over the water in a lush green arch. Pairs of parrots flew overhead, talking to each other, while the screaming piha bird produced its ventriloquial two-note call that seemed to emanate from every tree. In Tarzan movies, the dubbed-in sound track always included a screaming piha's piercing call, evocative of the universal jungle. In the distance, howler monkeys produced their high-volume whoops. Elongated bromeliad leaves and orchid pseudo-bulbs festooned host trees in increasing numbers as we motored upstream.

In the upper reaches of the Cueiras, we found the water level too low for the Alyson's five-foot draft and switched to the canoe. Another hour of motoring upstream brought us to the jungle home of Heidi Mosbacher, the sloth lady.

Heidi came to Manaus 16 years ago from Germany as part of an agricultural mission and stayed out of interest in and concern for the welfare of the three-toed tree sloth. In the tradition of Dian Fossey and Jane Goodall, but without the financial support they enjoyed, Heidi moved deep into the rain forest of the River Negro watershed and built a tiny thatched hut beside the Cueiras. There, all alone, on an income that would amount to poverty for even the most deprived cabocla, she began a study of the life cycle and behavior of one of the jungle's stranger mammals.

A tree-dwelling, leaf-eating creature about the size of a small house cat, the sloth has but one defense against the many carnivores that hunt it: immobility. A sloth can remain utterly motionless for hours. With its brindle coat greened with moss, appearing as nothing more than a part of the tree in which it sits, it becomes invisible to most eyes. Its movements, when it makes them, are so slow as to be almost imperceptible.

Yet sloths, Heidi pointed out, are a widely misunderstood species about which little is known and much misinformation abounds. Contradicting various claims, she said that sloths "can move quickly if they have to. They are not stupid. They communicate. They are affectionate. They look after each other. They are clean. They have good eyesight, sense of smell and hearing."

"People must learn," she said, "that sloths are an important species worthy of protection."

With Marica the sloth dangling contentedly from one shoulder, Heidi waved goodbye as we turned the canoe downstream toward Manaus and civilization. Her self-imposed and unsung quest struck me as an analogue for all of Amazonia. If durable sloths can't make it, can other species long survive?

Downs Matthews' most recent book for young readers, "Arctic Foxes," is scheduled to be published by Simon and Schuster next year. He is also the author of "Polar Bear" (Chronicle Books) and several other books for children, including "Polar Bear Cubs" and "Arctic Summer" (Simon and Schuster).

To see the animals of the Amazonian rain forest, you'll need water-borne transportation and knowledgeable guides to take you to them. Among the possibilities:

Amazon Adventurers (Suite 200, National Press Building, Washington, D.C. 20045, 202-508-8660) arranges tours of the rain forest for groups, individuals, teachers and women-only groups. The weeklong Rustic Native package costs $1,759, including round-trip air fare to Manaus, hotel accommodations, three days in a jungle camp, meals and a boat excursion of the Negro and Solimoes rivers.

Special Expeditions (720 Fifth Ave., New York, N.Y. 10019, 800-762-0003) offers cruises of the Upper Amazon in the 80-passenger Polaris. A nine-day trip beginning Wednesday and again Oct. 27 leaves Iquitos, Peru, and descends the River Maranon and the River Solimoes to Manaus, then returns on the same route. The price starts at $3,710 per person, including meals, accommodations and excursions on inflatable boats.

Alternatur (Rua Costa Alzevedo No. 9, Sala 2033, Centro Manaus, Amazonas, Brazil, CEP 69,000, 011-55-92-232-5541) books riverboat tours out of Manaus for groups of up to 20.

Amazonia Expeditions (Rua Madrid No. 13, Quadro 4, Campos Eliseos, Manaus, Amazonas, Brazil CEP 69,045, 011-55-92-238-3631) offers intimate, custom-tailored expeditions for groups of up to 12 on the riverboat Alyson. Owner Charles F. Grice, an American expatriate, specializes in visiting seldom traveled waterways of the Amazon tributaries and will accommodate special interests of guests.

Victor Emanuel Nature Tours (P.O. Box 23307, Austin, Tex. 78764, 800-328-8368) conducts birding tours to various destinations in Amazonia. The 12-day Amazonian Brazil Tour departs from Manaus Jan. 13 and costs $2,995, including a four-day boat trip on the River Negro, meals, accommodations and guides.

Rio Amazonas Turismo Ltda. (Rua Silva Ramos, 41, Manaus, Amazonas, Brazil CEP 69,000, 011-55-92-234-7308 or 011-55-92-232-4160) offers River Rio Negro tours, but its best attraction is the Ariau Jungle Tower Hotel. Not far from Manaus, the Jungle Tower consists of 40 rooms built at five levels around a tower providing a panoramic view of the surrounding jungle. Animals are easily seen and photographed close by. A three-day package costs $340 a person.

Invitational Tours (P.O. Box 891, Greenville, Miss. 38702-0891, 601-335-2444) specializes in tours for small groups of nature photographers and bird watchers, and visits Amazonia once a year in May. Eleven-day tours originate in Miami and include seven days on a riverboat and three days at the Ariau Jungle Tower Hotel. Cost is $2,800, including transportation from Miami, meals, accommodations and guides.

GETTING THERE: Transbrasil offers a direct flight between Washington and Manaus with one stop in Brasilia. The round-trip fare is $1,078 with restrictions. Varig offers a nonstop flight between Miami and Manaus. The round-trip fare, which includes the flight between Washington and Miami on another carrier, is $1,300 with restrictions.

INFORMATION: Brazilian Embassy, Cultural Section, First Floor, 3006 Massachusetts Ave. NW, Washington, D.C. 20008, 202-745-2804.

-- Downs Matthews