Alphonso L. Harris, one of the 10 young persons on trial in the slaying of Catherine L. Fuller, took the witness stand in his own defense yesterday and testified that he did not kill the 48-year-old Northeast woman and that he was with a friend and a girlfriend the afternoon Fuller was beaten to death.

The 23-year-old Harris, the first of the accused to testify, also told the D.C. Superior Court jury that he did not know some of his fellow defendants and that most of the others he knew casually.

One of the prosecution's key contentions is that the 10 defendants regularly congregated at a Northeast park and were gathered there Oct. 1, 1984, when Fuller passed by and the group decided to rob her.

Harris, also known as Monk, testified that he rarely visited the park, located a few blocks from the alley where Fuller was attacked, and that he only jogged through it on the way to a girlfriend's house on the day Fuller was killed.

Speaking calmly and often breaking into a smile, Harris described Oct. 1, 1984, as an uneventful day spent getting parts for his Camaro car, watching a James Bond movie, taking cookies to a girlfriend's house and sitting outside in a car with another friend.

When his lawyer, Michele Roberts, asked, "Did you in any way assist the robbing, beating or killing of Catherine Fuller," Harris resolutely answered "No."

Two prosecution witnesses, who have pleaded guilty to killing Fuller, have testified in the nearly three-week trial that Harris beat and kicked Fuller in a Northeast alley. One of them, Calvin L. Alston, testified that Harris said "shove it some more" when Fuller was attacked with a pole.

The three other eyewitnesses said they did not see Harris at the scene of Fuller's beating.

Earlier in the day, as defense testimony got under way, four witnesses, including Harris' former girlfriend, Carolyn Powell, 22, and Harris' mother, Helen Brooks, gave testimony that substantiated major parts of Harris' account.

During a terse cross-examination, Assistant U.S. Attorney Jerry S. Goren attempted to discredit the credibility of Harris' four witnesses. He showed that there not only were discrepancies between the witnesses' testimony but also in their accounts to a grand jury over when and if certain events took place.

Time is a crucial factor in the case. A medical examiner testified that Fuller died between 4:30 p.m. and 6:30 p.m.

Harris said yesterday that he left a friend's house about 4 p.m. and arrived at Powell's house on Maryland Avenue NE about 10 minutes later with cookies and a bowling bag that contained an electrical harness for a car. He said he spent the next hour and a half in Powell's house or in a car outside of her house. He said he arrived home about 6 p.m.

Goren showed some anger yesterday as he tried to break down Harris' testimony by showing that parts of his account were not true.

One of the key parts of Harris' testimony involved the electrical car harness that he said he had taken from an abandoned car earlier in the day. Almost all of Harris' witnesses mentioned the harness in their accounts and said Harris had either shown them the harness or discussed it with them.

When asked to describe the abandoned car from which he got the harness, Harris said it "was not like a car. It did not have a hood or trunk or doors or wheels." Several minutes later, Goren displayed two large photographs of a car that Harris said he thought "was the same one." The car had both a hood and a trunk.

Harris remained composed throughout much of his examination, even as he acknowledged a 1981 conviction for carrying a dangerous weapon.

Following his testimony, Harris' lawyers, Roberts and Corinne Schultz, seemed pleased by their client's appearance. Other defense lawyers said they felt Harris "held up well" and that Goren "did not hurt him too much; that's all you can hope for with an alibi defense."

Harris' testimony continued a debate among the 11 defense lawyers over alibi defenses.

Alibis are among the least favored defenses of lawyers because of the ease with which conflicts in testimony can be shown.

One defense lawyer said yesterday that another problem with an alibi defense in a case like this is that Fuller's beating took place in a short period of time.

"Nearly all these kids lived only a couple of minutes away from the alley," the lawyer said. "They could have slipped in and out of there in five minutes and no witness is able to account for every minute exactly."