In the 16 years since Bubba the grouper was dropped off in a bucket on the doorstep of the Shedd Aquarium, he has turned from a girl into a boy and grown to a whopping 140 pounds.

That in itself makes a pretty good story, but now Bubba has made marine veterinary history. Diagnosed last year with cancer, he is believed by his keepers to be the first fish to undergo chemotherapy.

Earlier this month, a healthy Bubba was slipped into his new home, the 400,000-gallon main pool of the Shedd's new $43 million Wild Reef gallery.

As a member of a dwindling species, Bubba would be highly valued in any aquarium. But of the many thousands of fish at the Shedd, Bubba is beloved by the aquarium staff and regular visitors for his history as a foundling and for his persnickety nature. Nobody wanted to let the sick fish slip away without a good fight.

After the diagnosis of connective tissue cancer, the Shedd brought in experts in veterinary oncology and surgery to cut a tumor from Bubba's forehead; then they used a course of chemotherapy, a first in the annals of fish husbandry anywhere in the world, according to the aquarium.

A pleased Mark Schick watched as Bubba explored the Pacific reef habitat alongside dozens of sharks and hundreds of colorful tropical fish.

"From day one of the planning of this [gallery], we had been thinking this is where Bubba belonged," said Schick, collection manager of the Wild Reef. "It is so incredibly satisfying to see him healthy and taking his place in there."

Four feet long and growing, Bubba is a Queensland grouper, a species that in nature is the biggest fish inhabiting the shallow waters of coral reefs in the South Pacific.

Nobody knows where he was born, but it is likely that he was netted as a baby in the wild by fish collectors who mistook him as belonging to a much smaller species. Someone then bought him in a pet shop, probably when he was no more than 2 inches long.

"When he got to be 10 inches long," Schick said, "the people who owned him probably realized he was going to be much too big for their aquarium tank. Somebody on our staff found him in a bucket on our doorstep in 1987 with a note asking us to give him a good home."

The Shedd normally does not accept castoff fish, but the staff recognized him as a Queensland grouper, a species fast disappearing in nature because of the destruction of coral reefs and because they make delicious eating.

Bubba started life as a girl, as do most groupers. While growing up at the aquarium, he turned into a male fish, a common phenomenon among many fish species known as "protogynous hermaphroditic." It is nature's way of waiting to see how many male fish are needed for successful breeding in a given population, then meeting that need.

For his first 11 years at the Shedd, Bubba swam in a coral fish exhibit in one of the original galleries of the 70-year-old aquarium.

The staff took to calling him Bubba because of his hulking appearance and manner. With a mouth big enough to swallow a soccer ball whole, he sucks in fist-size balls of fish meat when fed by keepers.

"He is such a character," said Rachel Wilborn, one of his keepers. "He is so curious, always coming around to see what you are doing. If you give him a food item that he doesn't like, he spits it right back at you, then looks you right in the eye, waiting to see what else you can come up with."

In 1998, the Shedd closed the gallery he lived in and ripped it out to make room for a new Amazon River exhibit hall. He was taken off the display at that time.

"That's when we found out how popular he was," said Shedd spokesman Roger Germann, "because we started getting letters from people saying they couldn't find Bubba in their last visit and wanted to know what had happened."

Several years ago, when the Shedd began planning an addition to house a major exhibit on Pacific coral reefs, the staff was very happy to have Bubba, whose species is a top predator in such wild habitats. They are hard to find in the wild or in zoos.

In August 2001, trainers became alarmed at the appearance of growths on Bubba's forehead.

"He looked like he had pink pimples erupting on his head," said keeper Heather Thomas. "The veterinary staff first treated them as a bacterial infection, but they did not respond to antibiotics and turned into even bigger pustules."

A biopsy early in 2002 showed nothing, but as the condition worsened, a deeper biopsy showed that the growths were cancerous tumors. Last November, surgery and chemotherapy conducted by a team of specialists invited by Shedd veterinarian Natalie Mylniczenko seemed successful, but in February the tumors returned.

"We were doing something nobody, as far as we can tell from reading the literature, had ever tried to do before," Mylniczenko said. "When dealing with unknowns and exotic animals, you have to extrapolate from animals you do know.

"In our first attempt, we erred by cutting as little as possible. Last March, we tried the procedure again, but cutting out much bigger margins around the tumors so that we were sure we had it all."

The surgery was done in a narrow trough of water after Bubba was removed from his pool and anesthetized. The chemotherapy solution was injected into the tissue near the surgery area, and surgeons sewed in a patch of pigskin to get his own skin to grow back.

As he recovered, keepers gradually let other fish and sharks into his holding pool to prepare him for busy reef life.

Two months ago, a golden trevally fish slipped into the holding pool from the reef exhibit and became Bubba's constant companion, a partnership often seen in the wild. The 5-inch fish eats particles of food that escape Bubba's mouth during his messy meals.

Other than a big dent in Bubba's head, he seems no worse for the wear.

"If you want to know why we went to all this effort for a fish," Wilborn said, "all you have to do is look into his adorable face. We did it for Bubs because he is such a cool fish."

Bubba, a Queensland grouper, swims with a tiny golden trevally at Chicago's Shedd Aquarium.