Carole Bland Jackson, a former model and onetime friend of Hazel Diane "Rasheeda" Moore, testified yesterday that she saw D.C. Mayor Marion Barry sitting on the side of a bathtub, snorting cocaine with Moore on the evening of the Riverfest festival in the summer of 1986.

Jackson, an admissions officer for a trade school in Norfolk, said the incident took place at the Tiber Island apartment complex in Southwest Washington, where she, Moore, the mayor, Barry friend Gaylord Tissueboo and another man had gone after the Riverfest boat ride. Moore, Barry and the other man disappeared into the bathroom shortly after they arrived, she said, and then Moore came back to ask her if she wanted to "get high."

"I went back into the bathroom, and I noticed the mayor and Rasheeda were sitting on the side of the bathtub," Jackson said. The four of them then took turns snorting the cocaine through a straw, she said.

Jackson's testimony about the Tiber Island incident corroborates testimony by Moore. Jackson is one of three witnesses -- the others are Moore and former D.C. employee Charles Lewis -- who have testified that they saw Barry use cocaine. So far, seven witnesses have testified that they used illegal drugs in the mayor's presence.

The identity of the fourth person allegedly in the bathroom that night became public during cross-examination of Jackson by Barry co-counsel Robert W. Mance, when Mance referred to "Mr. Rivers" in a question -- a reference to former D.C. Department of Human Services director David E. Rivers.

Rivers is on trial in the same courthouse for allegedly steering department contracts to his friend, John B. Clyburn, who is also on trial, and friends of Clyburn's. Closing arguments in that 3 1/2-month trial started yesterday and are expected to continue today. {Story on Page D6.}

The mention of Rivers's name prompted an objection from Assistant U.S. Attorney Judith E. Retchin, followed by a bench conference with Mance and U.S. District Judge Thomas Penfield Jackson. Jackson sealed the transcript of that bench conference.

Before Carole Jackson took the witness stand yesterday, prosecutors won permission from Judge Jackson to show jurors a videotape of a Sept. 26, 1986, news conference in which Barry labeled as "crybabies" those who objected to the use of "scam" operations in which undercover officers pose as drug buyers or sellers in order to make arrests.

"With this drug epidemic, got these crybabies who say, 'What about the rights of those who are going out to buy drugs?' " Barry said on the videotape. "As far as I am concerned, they ought not to be out there buying drugs in the first place, and there would be no question about their rights." Mance objected to introducing the videotape into evidence, saying that it was irrelevant and inflammatory.

But the judge agreed with Retchin, who argued that Barry's knowledge of how undercover police operations worked explained why he acted "guarded and suspicious" during the FBI sting on Jan. 18 at the Vista Hotel.

Retchin also argued that the videotape corroborates Moore's and Lewis's testimony that Barry was continually fearful that police were following him. Lewis said Barry unscrewed lightbulbs out of fear they were surveillance devices.

Jackson said that she had known Moore for years but that they had a falling out in the summer of 1986, while working for Moore's city-funded summer youth program, Project Me. Mertine Moore, Rasheeda Moore's sister, was also involved.

Jackson said she felt she was doing all the work. "We were all making the same amount of money," she said. "But there were occasions when Rasheeda would not be there during the middle of the day, and Mertine never did come."

Jackson testified that on the same evening of the Riverfest boat ride, Barry made sexual advances to her on the patio of the apartment where they used cocaine.

Jackson also testified that a few days before she snorted cocaine with Moore and the mayor, she saw Barry sitting on the side of a bathtub in Tissueboo's house, sniffing. She and Moore snorted cocaine in the mayor's presence that evening, she said, but she said she did not see Barry use the drug.

Jackson's testimony was followed by almost three hours of what lawyers call "chain of custody" evidence. A prosecutor must prove that a particular piece of evidence is in roughly the same condition as it was when it was seized by police, by calling as a witness everyone who has had custody of the evidence since it was seized.

A chemist, FBI agent Thomas P. Lynch, testified that the crack seized at the Vista was found to be 93 percent pure, while the crack taken from Barry's jacket was found to be 92 percent pure. Under questioning by Roberts, Lynch said the difference of one percentage point did not mean that the two samples were scientifically distinguishable.

Other items were found in Barry's possession, including several photographs, business cards and paper with written notations.

Although most of the afternoon's testimony was tedious, there were some lighter moments. FBI evidence technician Frances Henning drew laughs when she told Barry's chief attorney, R. Kenneth Mundy, what the photos showed: potholes. "Very large potholes," Henning added, after a pause. "That was your line," Mundy said. "I was waiting for it."

Staff writer Barton Gellman contributed to this report.