Lawyers for D.C. Mayor Marion Barry are trying to locate a retired Army enlisted man who they believe could cast doubt on a key allegation against the mayor, defense lawyer Robert W. Mance said yesterday.

The potential witness is Albert E. Benjamin, 55, who lives in the Virgin Islands and who sources said has told a grand jury that he saw no cocaine on a St. Thomas cruise he took with Barry in 1986.

That testimony could be critical when jurors in Barry's drug and perjury case have to decide whether to believe Charles Lewis, who said Barry used cocaine and crack with him at a Ramada Inn in 1988 and in the Virgin Islands in 1986 and 1988.

The problem for defense lawyers is that the reclusive Benjamin apparently has no interest in helping Barry.

"He could be the one to establish the reasonable doubt we need to obtain an acquittal on the felony counts," Mance said.

Although Benjamin's potential testimony involves only a portion of Lewis's allegations against Barry, Mance said he hopes it could have the effect of calling Lewis's entire story into question.

Mance and his partner, R. Kenneth Mundy, have retained a Virgin Islands lawyer and private investigator to try to find Benjamin.

Prosecutors have not subpoenaed Benjamin. As required by federal law, prosecutors told the defense that Benjamin is someone who could provide "exculpatory" testimony -- that is, testimony helpful to Barry.

The three felony charges against Barry, alleging that he lied to a federal grand jury about his knowledge of Lewis's drug involvement, are the only charges in the 14-count indictment that carry an almost certain prison sentence.

Benjamin told The Washington Post in a brief telephone interview on Jan. 17, the day before Barry's arrest at the Vista Hotel, that he had seen no drugs on the 1986 boat trip he and his wife took with Barry, Lewis, Lewis's friend Jonetta Vincent and Hazel Diane "Rasheeda" Moore.

After a few moments of conversation, Benjamin became agitated by a reporter's questions, declined an in-person interview and ultimately denied that he was the Albert Benjamin who had talked to the FBI and testified before a grand jury about Barry.

"You have the wrong Albert Benjamin," he said.

If defense lawyers can produce Benjamin at the trial, he could add another possible contradiction to Lewis's testimony about cocaine use on the 1986 cruise. Already, Vincent and Moore -- Barry's chief accuser -- have testified that they saw no cocaine use on the boat, although both women said Barry used marijuana on the cruise.

The alleged marijuana use is expected to be disputed by D.C. police Lt. Ronald Harvey, a member of Barry's security squad who accompanied Barry on the boat.

As if in anticipation of Harvey's expected appearance as a defense witness, Assistant U.S. Attorney Judith E. Retchin has elicited testimony from several witnesses, including Vincent and Moore, about Barry's security detail.

Retchin asked Vincent whether Harvey was in a position to see the marijuana. Vincent answered that he was. Did he do anything to prevent it, Retchin asked. Vincent replied that Harvey did not.

Lewis has testified that he introduced Barry to Benjamin and to Benjamin's wife, Carmen Acosta Benjamin, a longtime friend of Lewis's. Lewis testified that Barry sampled some of Carmen Benjamin's bullfoot soup. Lewis told authorities that Barry wanted the island delicacy because it is reputed to be an aphrodisiac.

According to sources, it took several days for FBI agents and D.C. police to locate Benjamin last year and serve him with a grand jury subpoena.

Benjamin lives in a remote portion of the Frydenhoj area near the eastern tip of St. Thomas. Benjamin's telephone number is not listed, and his home is not on the regular mail delivery route. The letter carrier assigned to Benjamin's route told The Post in January that he did not know where Benjamin's house is located. He guessed that Benjamin receives his mail through a post office box in the St. Thomas city of Charlotte Amalie.

Testimony is expected to resume today in Barry's trial with further cross-examination of Lydia Pearson, a crack dealer who testified Friday that she sold drugs to Barry more than 25 times in 1988.

Over the weekend, Barry, his wife, Effi, and son, Christopher, traveled to Atlantic City, where they relaxed, attended church and got away from the pressure of the trial, said Barry spokeswoman Lurma Rackley.