At first, they were calling it "Naked Sun" and emphasizing the heat and the exotic tropical setting. Then, when they decided that the jewels and the robbers who steal them deserved more "attention, they began to call it "Greed."

But if Carlo Ponti and company had really wanted an accurate title for the movie they have been shooting in this chic but isolated Brazilian resort town, they'd have settled for something like "Jaws Goes to Rio."

Never mind "Blonic Man" Lee Majors, who plays the leader of the jewel thieves. Never mind Margaux Hemingway, Marisa Berenson, Karen Black and all the other high-priced human talen. The real stars of this $6-million disaster flicks are the extras - hundreds and hundreds of Brazilian piranhas, as alive and as nasty as the day they were first fished out of the Amazon.

As Majors explained it, "Greed" is "an action-adventure picture in which a lot of people get eaten up by piranhas." The pilot - what there is of it - is simple: A gang of jewel thieves stash their stolen goods at the bottom of a Brazilian lake but then find that, thanks to a tidal wave and a burst dam, the lake has been invaded by a horde of voracious, man-eating fish that won't allow the retrieval of the loot.

But who needs to bother with a plot when you're got the most vicious fish in the world working for you for room and board? There was only one problem: The Brazilian piranhas lived up to their reputation and proved to be more temperamental and uncooperative than even the most difficult of Hollywood stars.

Take the first batch of piranhas, who were lured out of the Rio Araguaia, an Amazon tributary, with tasty bait and flown down to the set of "Greed" aboard a specially chartered plane. On the trip from their Amazon home, most of the fish insisted on eating through the plastic containers in which tye were being shipped, and by the time the plane landed, all but a handful of the aquatic passengers had expired.

That lesson learned, the next batch of killer fish was brought to the set, a secluded cove on the Atlantic Coast some 125 miles west of Rio de Janeiro, safe and sound in glass and metal cylinders. Everything was fine until the piranhas were poured into a tank for preliminary filming.

That's when they started to attack and eat one another.

"They devour each other with a ferociousness and rapidity that has to be seen to be believed," said Alex Ponti, son of producer Carlo Ponti, son of producer Carlo Ponti and production supervisor of the film. "We've lost 200 or 300 of them that way."

"You put a group of them in water and they immediately go for each other," said one of the production team's Brazilian associates. "The weakest ones get gobbled up first, of course, but when it's all over, you've got nothing lelft but bits of bone and fin. And blood, lots of blood."

At a reported price of $150 a fish, that's not peanuts. And to that the resulting shooting delays and you've got something of a crisis for a movie that, in the words of leading-man Majors, was "a $20 million picture that they're trying to shoot for a nickel and a quarter."

To save the day, the Pontis hired a fish specialist, who came to be known on the set as "the piranha anha - but his duties actually did include rubdowns for flagging fish.

When it was time to shoot a scene in the piranha pool, Maluf stood by attentively. If one of te fish showed signs of tiring - which would immediately cause the others to turn on it and initiate the general bloodshed - Maluf Yawked it out of the water by its tail and massaged it gently until he thought it had regained its strength and could return to th fray.

Perhaps even more dangerous is the task that will be entrusted to one of the cameramen when location shooting ends. Because the piranhas have not responded as expected in captivity, Ponti is sending a crew up to the Araguaia, where underwater scenes will be shot right in the piranhas" native habitat.

"Nothing is going to happen to them," said Alex Ponti, with only the slightest trace of doubt in his voice. "These are guys who have worked with Jacques Cousteau. They know what they're doing. They're not worried."

Neither was Lee Majors, for that matter. He explained that he and Black have "two key scenes with the piranhas," including one in which the fish put a grisly end to the character he plays. But in both scenes. The Bionic Man and his leading lady will be swimming in a tank that is protected from the piranha pool by a sheet of invisible glass.

Not that Majors - like Black, Berenson and Hemingway - doesn't have his complaints. "Greed" is his first experience making a film overseas, and the $6 million Man had mixed feelings about his $6 million movie.

"This is not exactly the script we wanted," he said, swatting one of the multitude of mosquitoes swarming around the set. Preparing for his role was easy: "I took all my shots and brought my diarrhea medicine."

"This is such a godforsaken place," he complained as he killed time between scenes with the makeup man and the godyguard who accompanied him from the United States. "With American actors and an Italian and Brazilian crew, there's a real language barrier and a real lack of organization. The only thing that could possibly be worse than this is out in the jungle oil the set of 'Apocalypse Now.'"

Majors had been placated - at least partially - by being installed in an elegant house that has its own pool and a private beach - both piranha free, of course. He had been spending most of his free time there, "playing Ping Pong with the guys" - and talking on the phone with Farrah Fawcett-Majors.

"I feel cut off," he said. "Cut off from my wife, cut off from my family. I even feel cut off from my agent. They give us a per diem here, and mine just about covers my phone bill.

"It costs like $6 or $7 a minute to call the States from here," he added, shocked. "At a half an hour a day, that mounts up."

In compensation for Farrah's absence, Majors had been introduced to famed Brazilian plastic surgeon Ivo Pitanguy. The doctor owns an island near here - also piranha-free - and Majors spent a weekend there, lunching on Pitanguys yacht and playing tennis with Pitanguy and a pair of Brazilian millionaires.

Did that mean, as had been widely reported in the Brazilian press, that The Bionic Man would be going into Pitanguy's clinic for body work when the filming of "Greed" ends? "Not that I know of," he said, annoyed. "The doctor is a remarkable man, a brilliant man, but we are just good friends."

But he was intrigued: "What is it they say I'm going to have done?" he wanted to know. Crows' feet and wrinkles would be removed from around his eyes, he was told.

He appeared relieved, "Naah, I wouldn't want to do that," he said, swatting another mosquito. "I like crow's feet. They give a face character."