The woman in front of the packed University of Maryland classroom, her five-foot frame graced by a "Waikiki Aquarium" T-shirt, was telling a fish story of colossal dimensions -- by the sound of things, a whopper.

"The largest shark in the world is the whale shark," explained Eugenie Clark, waving her sinewy arms in an appropriately big sweep. "An adult whale shark can be 50 or 60 feet long and weigh as much as 10 tons. But the only way to really get a good look at it is to get on it and ride it. So last year, that's what I did."

The hundred or so students in Zoology 181 reacted with wide-eyed wonder or smiled skeptically as Clark recounted her tale. Before long, as she told how she nearly slipped past "teeth like a baby's fingernails" and down the leviathan's throat, passersby from the corridor began crowding the doors. They stayed to listen as Clark told of grabbing hold of the dorsal fin -- "my arm felt pulled out of joint" -- and climbing onto the shark's back, only to slide to the tail and be "swept in an arc as wide as this room" before plunging into the deep near Baja California.

Underwater photographer David Doubilet witnessed the spectacle and recorded it for posterity. "She just grabbed the dorsal fin as the monster swept by, and her tank slipped out of its harness," he says. "But she held on, slipping farther and farther down the tail as the fish swim away. I figured that was the last I'd ever see of the good doctor."

"I didn't want to ride them as much as I wanted to observe them," Clark said later in her office, a room she said a plankton-eating whale shark could easily swallow whole. "But the experience, I guess, was one of the most memorable of my life."

For Genie Clark, whose study of sharks and their habits has given her a worldwide reputation, that's saying a lot.

A professor of ichthyology at Maryland's College Park campus for the last 13 years, she has lectured on fish behavior everywhere from England to China, written two popular books as well as scores of magazine articles, taught sharks to ring a bell for food and the crown prince of Japan to skin-dive, and spearheaded, with the encouragement of the late Egyptian President Anwar Sadat, a politically sensitive campaign to protect the Red Sea from enterprising fishermen.

Through it all, she has managed to raise four children with the help of a governess, making sure all of them swam before they walked. She has married and divorced four husbands (a Japanese photographer, a Greek shipping magnate, an American writer and a Russian neurologist, respectively), and achieved a degree of celebrity most scientists never approach. Next January, she'll star in a nationally aired television documentary produced by National Geographic, a magazine for which she frequently writes.

"Diving with Genie is one of the most amazing things I've ever done," says Doubillet, who has worked closely with Clark on National Geographic stories for the last 10 years. "She's so hard to keep up with, with her tiny powder-blue flippers. She's certainly changed the way a lot of people look at the ocean -- not just eye-opening, but brain-opening. With her, every action of the ocean slowly begins to explain itself."

Still, Clark is not without her detractors.

"She's competent, she's a good swimmer and a good diver who's made quite a lot of observations," says Ernest Lachner, ichthyologist at the Smithsonian Institution. "And she certainly knows how to capitalize and attract attention. If the National Geographic wants to blow her up and the like, that's fine. But I don't remember when she gave her last comprehensive scientific paper. She's never been on the editorial board of the ichthyological journal. She's never been an officer of or served on any prominent committee.

"This is the hard-core work, the dirty work that takes up my time and everybody else's time so that scientists can survive and do research. She should do some of these things rather than being just a popularizer."

"Popularizer," the withering epithet that some scientists have hurled from time to time at the likes of Carl Sagan and Jacques Cousteau, isn't a new one to Clark. Yet the word still makes her boil.

"I've heard people say that Genie Clark is the scientist who publishes through goggle-like glasses her research results in the National Geographic," Clark, eyes flashing, complains. "But I'm working all the time on serious scientific research. I'm usually in my office till one in the morning, sometimes later than that."

"Look here," she said, rising from her chair and rifling through her file cabinets to come up with four recent papers, abstracts with titles like "Red Sea Fishes of the Family Tripterygiidae With Descriptions of Eight New Species." "These papers, nobody reads except the specialists."

As for scientific breakthroughs, Clark points to her recent studies of a variety of Red Sea sole that secretes a foolproof shark repellent whenever threatened. Clark believes the chemical might one day have a practical application in protecting human divers.

William Graves, Clark's editor at National Geographic, ascribes much of the criticism to "professional jealousy," as does University of Texas professor Clark Hubbs, chairman of ichthyology and editor of "Copeia," the field's preeminent journal.

"It's the old jealousy circumstance," Hubbs said. "I've heard some of my colleagues suggest that because Genie's name gets rattled around so hard, she should be kicked out of the American Society of Ichthyologists and Herpetologists. I say b---- to that. You don't downgrade someone because they get publicity. Those kinds of successes are a credit to the field."

Hubbs, who has known Clark since she studied under his father at the Scripps Institution of Oceanography 30 years ago, added, "Most scientists are prima donnas and Genie's no exception. But I think she's a nice prima donna."

At Maryland Clark is viewed with suspicion and misunderstanding by some of her colleagues, according to associate zoology chairman Howard Brinkley. "She doesn't make a large effort to convey to us as colleagues the things she does," he says. "She'd much rather do her work."

Giggling, shaking the brown bangs from her unlined face, Clark looks impossibly young for her age, an impression she enhances by favoring T-shirts and jeans, accented at times by shark-shaped buttons, and driving a fire-engine red Mercedes-Benz convertible to work from her home in Bethesda.

Even close associates, such as John Prescott the director of the New England Aquarium who has known her more than 20 years, are surprised to learn that she is 59 years old. "She is?" Prescott gasped.

Clark attributes her youthfulness to keeping in shape from diving -- "almost every thing I do seems an excuse for getting under water." As for her impressive collection of husbands, comparing to any she's made of exotic fish, Clark jokes, "I guess no man can stand me for long. I have no time for things like sitting on the sofa, watching television and holding hands. Unless, of course, "Jaws" is on."

These days, Clark devotes a lot of time to her pet project -- conserving the coral reefs of Ras Muhammad in the northern Red Sea, her favorite diving spot in the world.

"There are more coral species in that concentrated area than in any equivalent area in the world," Clark says. "But the environment is fragile, and the nylon fishermen's lines are breaking up the top-corals and destroying one of the most beautiful underwater areas."

Clark's plan to enlist the cooperation of four mutually hostile governments, Egypt, Israel, Jordan and Saudi Arabia, in declaring the area off limits to fishing, suffered a bad blow with the assassination of Sadat.

Clark has been a major figure in ichthyology, and beyond ichthyology, for more than three decades. In a field where men like the Smithsonian's Ernest Lachman labor years to study relatively obscure subjects such as North American freshwater minnows, Clark decided early to go for the glamor and discovered sharks.

By the time she became director of her own marine laboratory at the age of 33 (the Cape Haze in Sarasota, Fla., now called the Mote Marine Laboratory and still known as the center for shark research that Clark made it) she already had been on diving expeditions to the South Pacific, Bimini and Egypt, and had written a best-selling autobiography, "Lady With a Spear."

In the book, which has been translated into eight languages since it was a Book-of-the-Month Club selection in 1953, Clark tells how she grew up a New Yorker and at age 9 discovered fishes one Saturday when her mother took her to the old aquarium at Battery Park.

"All about me were glass tanks and moving creatures in them," Clark wrote. "At the back was a tank larger than the others, and the water in it was less clear, more mysterious. It was pale green and, a few feet from the glass wall that I looked through, it grew misty, as if there were no farther wall and the water just went on and on. Leaning over the brass railing, I brought my face as close as possible to the glass and pretended I was walking on the bottom of the sea."

These days, Clark vows to do the real thing "till I'm 90."