A former girlfriend of Levy Rouse testified yesterday that Rouse had blood "splattered" on his pants the night Catherine L. Fuller was beaten to death and that later he admitted he had "done the worst thing to that lady in the alley."

Eighteen-year-old Katrina M. Ward, who said Rouse is the father of her 4-month-old baby, recounted for a D.C. Superior Court jury a conversation she said she had with Rouse last December, several days before he was arrested in Fuller's murder.

"I called him a nasty dog because he had done something nasty to me . . . ," Ward testified in the trial of 10 young persons accused of beating Fuller to death. According to Ward, "He Rouse said, 'Nasty? I have done worse things than that.' He said, 'Girl, do you know I did the worst thing to that lady in the alley?' Then he looked at me and laughed."

As Ward quietly described the conversation, Rouse moved to rise from his seat and mumbled. Several U.S. marshals lining the courtroom lunged, and Rouse's attorney, Lillian McEwen, turned and stared at Rouse. Rouse stared briefly at Ward but subsequently rarely looked at her as she completed her testimony.

Ward's emotional testimony, one of the most dramatic parts of the month-long trial, occurred as prosecutors yesterday began to present evidence to rebut testimony from defense witnesses.

Ward testified yesterday that Rouse went to her house on the evening of Oct. 1, 1984 -- the day Fuller was killed. She said that when he arrived at her house there was blood on his "sky-bluish green" pants. When she asked him about it, Ward said, Rouse attributed it to someone "busting him on the nose" during a boxing bout. She testified, as she stepped down from the witness stand and pointed to her leg to demonstrate, that she did not see blood anywhere except for that "splattered" on his bottom right pant leg.

Rouse, who prosecutors say plunged a foot-long pole into Fuller's rectum, testified earlier in the trial that he did not participate in Fuller's death. Rouse, 20, told the jury that he had been with two friends roaming their Northeast neighborhood the day Fuller was killed.

Under cross-examination, Rouse denied that he had had blood on his pants, said he did not remember telling Ward he was in a fight, and he denied telling Ward about attacking anyone in the alley.

Ward testified yesterday that the subject of Fuller's death came up several days after the slaying when Rouse visited Ward in a hospital. Rouse, she said, confided that he had been in the alley but "didn't participate." Ward said it was not until three days before Rouse's Dec. 9 arrest that Rouse "laughed" about doing the "worst thing to that lady in the alley."

Questioned by Assistant U.S. Attorney Jeffrey Behm, Ward admitted that at first she did not tell anyone of Rouse's comments and that she "lied" during a grand jury examination. "He trusted me . . . ," Ward said.

Throughout the trial, Ward has been something of a mystery. She has been described as the lover of Harry Bennett, a key prosecution eyewitness who pleaded guilty to charges in connection with Fuller's death, and then as the mother of Rouse's child. Ward said yesterday she is now involved with Bennett.

In other testimony yesterday before Judge Robert M. Scott, a prosecution witness said that he and one of Fuller's sons, David, were waiting to catch a bus on Oct. 1, 1984, when they noticed police cars and an ambulance across the street near an alley.

George Raspberry, 15, told the jury that he crossed the street for a closer look but that David Fuller said he wanted to remain at the bus stop.

Raspberry identified a number of defendants as persons who had been milling around in the alley at the time, including some who testified they were not there.