A Prince George's County jail inmate was convicted yesterday of assault and battery and unlawful imprisonment of a fellow inmate last Nov. 27.

But Ronald P. Garnett, the first of 12 current or former inmates facing trial on sexual assault and related charges stemming from several separate incidents, may stand trial again on two charges of attempted sexual assault because the jury failed to reach a unanimous decision on those charges.

Garnett, 25, was one of three inmates charged with attempting to assault James A. Anderson sexually in a cellblock they shared. Anderson, who pleaded guilty last week to the first degree murder of a 17-year-old retarded youth, testified during the three-day trial that Garnett and another inmate beat him up and attempted to assault him sexually, with the help of a third inmate.

Jurors said afterward that during eight hours of deliberation, 11 of them had believed Garnett was guilty of the two attempted sexual assault charges, but one juror remained unconvinced. At 3 a.m., the jury notified the court that it could reach no verdict on those charges.

Circuit Judge Audrey E. Melbourne declared a mistrial on those two charges, meaning Garnett may face trial again. She will sentence Garnett on June 6 for assault and battery and false imprisonment, or holding someone against his will. The two crimes carry a 23-year maximum sentence.

Both defense and prosecuting attorneys agreed during closing arguments earlier this week that the credibility of witnesses was the key point in the case. Defense attorney Richard H. Sothoron Jr. argued that Anderson concocted his story of the sexual assault after reading a Washington Post series on rapes in the county jail, to win sympathy and pressure prosecutors into withdrawing death penalty papers filed against him.

Prosecutors agreed not to seek the death penalty as part of the plea arrangement in the Anderson murder case. But Circuit Judge Howard S. Chasanow, who approved the agreement and sentenced Anderson to 30 years in prison, testified as a prosecution witness that the sexual assault case had nothing to do with the plea bargain.

Anderson had nothing to gain by testifying that he was assaulted, and everything to lose, Assistant State's Attorney David Simpson, who prosecuted Garnett, argued. By the time the sexual assault case began, Simpson said, Anderson's sentence had been settled. Furthermore, Anderson "put himself in very grave danger" by testifying. "He's a snitch, probably even worse . . . ." said Simpson. "This is a man who is going to jail for a long time, and you are a dead man as a snitch."

One of the 12 male jurors interviewed yesterday said it was this factor that made most of his colleagues believe Anderson rather than Garnett. "Anderson has a lot to lose for being a snitch," the juror said. The main question the jurors faced was "who was the biggest liar," Garnett or Anderson, he said, and all but one juror believed Anderson. The hold-out, he said, "felt that both of them were lying," and he couldn't be sure of the nature of the assault.