JACKSONVILLE, FLA. (AP) _ Fugitive American financier Robert Vesco was identified today for the first time as a co-conspirator in the cocaine-smuggling trial of Carlos Lehder Rivas.

In arguing that a witness' testimony about his conversation with Vesco was admissible as evidence, U.S. Attorney Robert Merkle said the details were ''co-conspirator's testimony.''

No further details were immediately available about Vesco's role in the Lehder trial, since Merkle has consistently refused to discuss the case outside the courtroom.

Vesco is not named in the Lehder indictment but was mentioned by the first witness in the case as having a business interest in Norman's Cay, an island in the Bahamas.

The financier, who is believed to live in Cuba, has been a U.S. fugitive since 1974 on charges he plundered $224 million from an international investment fund.

Charles Kehm, former manager of Norman's Cay, the island the government claims Lehder used as his smuggling base, said he was threatened several times with death by Lehder and co-defendant Jack Carlton Reed when he refused to leave the island. Eventually, however, he did leave.

On a trip back to the island to retrieve his furniture, Kehm testified, ''We went to the main dock and I ran into Robert Vesco, of all people.''

After he told Vesco about problems in retrieving his personal property, Kehm said, ''He (Vesco) told me the best advice he could give me was to get off the island and keep my nose out of other people's business.''

On Wednesday, a Continental Airlines pilot admitted making three cocaine- smuggling flights for Lehder, conceding that he earlier lied on the witness stand.

Eben Mann said he was granted immunity from prosecution and advised of perjury recantation laws, which protect those who immediately correct false testimony, before admitting that he lied on Tuesday.

''I didn't tell all the truth, and I apologize to the jury and the court,'' Mann said.

The admission came in Lehder's trial on charges he smuggled 3.3 tons of cocaine into north Florida and Georgia from Colombia via Norman's Cay in the Bahamas from 1978 to 1980.

Mann testified Tuesday that all of his flights for Lehder were legitimate and that he had not carried any cocaine, but on Wednesday he admitted picking up contraband.

Mann told of three drug flights he made to Colombia in private planes in 1977-78 and said Lehder paid him $75,000 in cash.

Mann also said he lied when he denied that he and Lehder had discussed falsely registering aircraft.

''I was trying to protect myself and my friend,'' said Mann, who had testified about flights he and a partner, Sam Stewart, had made for Lehder.

Earlier in the day, a California disc jockey showed home movies of Norman's Cay in the Bahamas and testified about drug flights he made for Lehder.

Russ O'Hara, a Palm Springs, Calif., radio announcer who also received immunity for his testimony, said he began working for Lehder in 1978 as a co- pilot on flights made by John Finley Robinson and Lehder's co-defendant Jack Carlton Reed.

On his first trip to Norman's Cay, which the government claims was Lehder's smuggling base, O'Hara filmed the ''idyllic, quiet, sleepy little island'' and his girlfriend as well as Reed and Reed's girlfriend.

Lehder is being tried in a 1981 indictment alleging conspiracy, importation of cocaine and operating a continuing criminal enterprise. Co-defendant Reed, 56, of San Pedro, Calif., is charged only with conspiracy.

A separate Miami indictment accuses Lehder of being one of the leaders of the Medellin Cartel, which is said to be responsible for 80 percent of the cocaine smuggled into the United States.