Charles S. Turner Jr., one of the 10 young persons accused of killing Catherine L. Fuller, testified yesterday that Levy Rouse, his best friend and also a defendant, lied when he told the jury that the two were together the afternoon Fuller was beaten to death.

The 21-year-old Turner also contradicted the testimony of his key defense witness, his mother, who said earlier yesterday her son was with her when Fuller was killed on Oct. 1, 1984.

Turner told the D.C. Superior Court jury yesterday that on that day he remembered getting up, going to a park for a short period and then returning to his grandmother's home where he took a nap. Turner said he did not see Rouse or his mother until later that night.

Beverly Alson, Turner's mother, testified that she was at her daughter's house smoking marijuana for most of the afternoon and that her son was with them. Rouse has told the jury that he, Turner and another friend spent the day roaming the neighborhood.

But during questioning by Assistant U.S. Attorney Jerry S. Goren and Rouse's lawyer, Lillian McEwen, Turner acknowledged his memory of Oct. 1, 1984, was poor and that he could, in fact, remember very little of it.

Turner finally said, "I don't really remember doing anything" after repeatedly answering "I don't remember" or "I don't know" to questions about what he did that day.

Judge Robert M. Scott turned to Turner and said, "you have no recollection what you did all day long before being" awakened later that night by the doorbell? Turner said he did not.

Following Turner's testimony, a number of courtroom observers questioned Turner's alibi defense, considering that his testimony contradicted that of his key witness and that he was able to recall so little about the day of Fuller's death.

"There is only one thing that can sink you faster than a strong government case -- it's a weak alibi," said one defense lawyer. But another defense lawyer said Turner was "almost sympathetic" because he remembered so little.

Turner is the fifth defendant to base his case on testimony that he was not present in the alley or abandoned garage where prosecutors allege the 48-year-old Fuller was beaten and kicked to death by a group of young people during an attempted robbery of coin purse. Two other defendants have rested their cases without presenting a defense, and at least two of the three defense attorneys yet to present their cases have indicated they plan to present testimony that their clients were elsewhere when the killing occurred.

Turner has been identified as a participant in the beating by most of the prosecution eyewitnesses, including some who testified Turner held one of Fuller's legs while Rouse, 20, assaulted her with a pole.

In a statement before the jury yesterday, Lloyd Elsten suggested that his client was implicated in the case because of his close relationship with Rouse. Defense lawyers have tried to disassociate their clients from Rouse and have argued to the judge that Rouse's alibi was so "incredible" that it had harmed other defenses.

Turner acknowledged yesterday that some of Rouse's account of what he and Turner did the day of the killing was accurate, but that the events had happened at night and not in the afternoon as Rouse had testified. Turner said Rouse and another friend came to his house later that night and told him that someone had been killed in the garage. He said the trio then visited the alley. Contradicting another part of Rouse's alibi, Turner denied that Christoper D. Taylor, Rouse's key witness, was with them.