Richard A. Byles, a lanky zoologist, grabbed the 123-pound sea turtle by its arched shell, hefted the brownish beast to the edge of his research boat and tossed it into the calm green waters where the York River enters the Chesapeake Bay.

The loggerhead turtle bobbed along the surface momentarily. Then it plunged toward the depths. "She's gone," Byles said.

The turtle's graceful dive marked the start of a scientific tracking expedition meant to uncover some of the mysteries of one of the earth's ancient and imperiled creatures. The meandering voyage also provided a glimpse into the painstaking and interminable tasks scientists are undertaking as they search for ways to preserve rare and often poorly understood animals and plants from extinction.

Sea turtles, whose remaining species are thought to date back 80 million to 100 million years, have been slaughtered for food, used as fish bait and turned into jewelry, ornaments and apparel. Turtle eggs, essential for the species' survival, are eaten. Their nesting beaches face increasing encroachment, raising fears by scientists and government officials that some sea turtle species soon may vanish.

"The turtles were here to witness the rise and fall of the dinosaurs, and they just swam along. But they may not survive man's rise," Byles remarked.

Moments after he heaved the loggerhead overboard, the turtle surfaced for a few seconds. It was not long enough for Byles to spot the reptile's thick head. But he heard the telltale beep of a waterproof radio transmitter fastened by a short cord to the trailing edge of the turtle's shell. "Hear that?" he asked. "She just came up." Then the animal dived again.

At the mouth of the York about 125 miles southeast of Washington, the water was placid beneath an overcast, hazy sky with only a few small fishing vessels in sight. A buoy's gong clanged in the distance. A blue heron gazed from a nearby perch. Menhaden jumped, and sting rays and jellyfish floated in the translucent, salty water.

Byles, wearing earphones, listened carefully to the repetitious ping-ping of a sonar transmitter, which emitted sound waves indicating the turtle's whereabouts. The loggerhead remained hidden beneath the surface, but affixed to its 2 1/2-foot shell were two scientific gadgets that allowed Byles to track it. A radio transmitter beeped whenever the turtle surfaced and a sonar transmitter pinged as the loggerhead swam underwater.

"There she is," said Byles, as the turtle came up for a breath of air and then dived again about 150 yards north of the boat. "Going for deeper water," Byles added. "We'll mosey in that direction and catch up with her." In a small log, he noted the turtle's initial movements as well as the temperature of the water. Then he pulled in the anchor and headed northward.

Spurred by a fascination with nature and a government-backed preservation drive, scientists such as Byles, a Virginia Institute of Marine Science researcher, are shedding new light on the five species of sea turtles that inhabit East Coast waters. All five -- the loggerhead and Kemp's ridley, which most often traverse the Chesapeake Bay, as well as the leatherback, green and hawksbill -- are listed as threatened or endangered under the Endangered Species Act, which mandates steps to prevent their extermination.

According to Jack Musick, a Virginia marine science professor, studies by institute zoologists in the last three years have shown the Chesapeake Bay area to be a vital summer habitat for thousands of sea turtles, chiefly subadults. Scientists do not know how to determine a sea turtle's age, but they believe most of those in the bay are more than 1 year old and less than 15 years. Apparently they come to the Chesapeake in summertime to feed on crabs and other marine life.

The institute's research already has punctured one long-cherished theory, the now-disputed contention that the bay region serves as a major nesting ground, where baby sea turtles would hatch if spared from intrusion.

"They don't use the area to nest in," Musick said. "In general, the results were negative. And that surprised us." The findings helped prompt the Interior Department to abandon its attempts to transplant turtle eggs to the Chincoteague and Back Bay wildlife refuges along Virginia's coast, where reseachers once hoped to establish nesting colonies.

Although sea turtles spend their lives almost entirely in water, females crawl ashore every year or so to lay eggs. Scientists theorize that bay-area beaches are rarely used as nesting grounds because there is insufficent seaweed in nearby waters to shelter the hatchlings and because the water may be too cold in autumn for baby turtles.

In addition, scientists say virtually no female turtles will hatch in the bay area. Without females, of course, the turtles could not propagate bay colonies. The sex of a turtle, scientists believe, is likely determined by the temperature of the egg from which it hatches. Cooler weather leads to male hatchlings and warmer temperatures to females. Since sea turtles are largely tropical animals, the Chesapeake area offers comparatively cooler weather and most offsprings, therefore, would be male.

Musick, Byles and other researchers are puzzled about the causes of turtle deaths in the bay. Some may drown after getting caught in large fish impoundment nets. Some may be struck by boat propellers and a few may be illegally killed for sale or by vandals. One recent theory holds that disease may play some role.

Richard E. Wolke, a University of Rhode Island pathologist, examined turtles found dead in East Coast waters, including the bay, and discovered a parasite in some of the reptiles' blood streams. This disease-causing blood fluke, Wolke said, may figure in their deaths or may weaken the turtles, leaving them susceptible to drowning. Possible remedies, Wolke said, may include injecting female turtles with drugs when they come ashore to nest or taking steps to deal with other animals that act as hosts for the parasites.

No one knows how many turtles dwell in the oceans and seas, but biologists say the number has dwindled. Peter C. H. Pritchard, a Florida Audubon Society vice president and authority on sea turtles, estimates that fewer than 850,000 adult females remain among the five East Coast species. (Only two other species exist.) These five species once numbered in the millions, Pritchard notes. The Kemp's ridley is closest to extinction, with only a few hundred females thought to survive.

"I have to catch up to her first. She's gotten away from me," Byles said as he started up his boat. Nine years ago, he gave up a computer career to study turtles. Now at 35, he was pursuing his dream. After three minutes, he stopped the motor to listen to the pinging of the sonar transmitter. Then he turned around. "She swam behind us."

For hours, Byles tracked the turtle, charting its movements and timing its dives. "She's not boogeying anywhere. She's staying in the same general area," Byles said. A mild drizzle fell for a half-hour, then stopped. The turtle zigzagged back and forth within about 500 yards of the spot where Byles had tossed it overboard. Once the loggerhead swam underwater for nearly 40 minutes before coming up for air. "That was a nice long time."

Plotting where turtles swim helps reveal the nature of their Chesapeake habitat, Byles said. Such knowledge may prove useful in protecting turtles from manmade threats, such as dredging and waterfront development. Timing turtles' breathing habits may help, Byles noted, in gauging how many turtles inhabit the bay. Bay researchers can count turtles only when they surface. They must estimate how many other turtles remain underwater and out of sight.

Throughout the summer, Byles will track the loggerhead and probably several other turtles. At times, he will stay out overnight, sleeping intermittently on an air mattress in his boat. Twice a month, he will crisscross the bay in a low-flying, single-engine plane to count turtles.

There are times of frustration. The other day, Byles spent four hours in a futile, 30-mile search on the bay for a different turtle. Perhaps its radio transmitter malfunctioned. Perhaps the turtle had disappeared from range. "She may still show up. I haven't lost hope," Byles said as he headed back to shore. "I'm disappointed."

The lure of nature prevails. "I was just awed by the size and the magnificence of the sea turtle," he remarked. "They swim through the water exactly as a bird flies through air. They flap their flippers just like wings. . . They use their rear flippers for guidance only. They use them as a rudder for steering."