Army Staff Sgt. Terry Lee Simmons, charged with murdering his girlfriend and their infant daughter in their Baileys Crossroads apartment, testified yesterday that his girlfriend, not he, killed their daughter.

Dressed in a crisp, olive-drab dress uniform, the 27-year-old sergeant sobbed uncontrollably at times during his 80 minutes of testimony as he told a Fairfax County jury of coming home from work on June 19, finding his 3-month-old daughter, Julie, dead and then struggling with his girlfriend, Caroline Yoo, 29, as she tried to strangle herself with a rope.

"Who killed Caroline Yoo?" asked Commonwealth's Attorney Robert F. Horan Jr.

"I guess I did," annswered Simmons.

"Sergeant," said Horan, "be fair to the jury. Do you believe she killed herself?"

"I don't know," answered Simmons..

The bodies of Caroline Yoo and Julie Simmons were found on Aug. 16 in a walk-in closet in the couple's $400-a-month, two-bedroom apartment after neighbors complained of foul odors and saw hundreds of flies inside its windows. When police entered the apartment, they found the badly decomposed bodies of the mother and daughter, who had been dead two months. There was a 3 1/2-inch bruise on Yoo's neck, and the child, found in a black garbage bag, had a 1 3/4-inch skull fracture.

In the closet were more than 20 different pine-scented air fresheners, mothballs, a pan of disinfectant and several incense burners, according to testimony at the Circuit Court trial, which will enter its third day today.

Simmons, who stayed in the apartment several days with the bodies, then with friends before going on a vacation to Canada, left Washington for Florida on Aug. 16, but turned himself in to military police at Fort Benning, Ga., on Aug. 20, according to testimony.

Court-appointed defense attorneys Jonathan C. Thacher and James Harney portrayed Simmons -- who was stationed at the Pentagon and had access to top-secret documents -- as a hard-working, career Army enlisted man who had been seduced and then badgered into a relationship with a dark, beautiful, Korean-born Army clerk-typist.

But after Julie was born, Simmons testified, Yoo felt "ashamed that she had had a child with an American GI and felt she could never face her parents again."

Simmons testified that Yoo neglected her child, refusing to change her diapers or feed her and "at least a dozen times" Yoo had wrapped a rope around her neck and threatened suicide. Simmons, who is married to a woman in Utica, N.Y., and has two children by her, testified that when he suggested he and Yoo put Julie up for adoption, Yoo said, "I'll kill the baby, I'll kill you and then I'll kill myself."

Under Harney's questioning, Simmons testified that he thought of committing suicide with a shotgun shortly before turning himself in.

Horan opened his cross-examination by pointing to a handkerchief that Simmons had brought with him to the stand and asking: "Did you anticipate you were going to cry for the jury?"

Prosecution witnesses portrayed Simmons as a man with a temper, who had asked a coworker to lie to Yoo for him and tell her that Simmons was working when he was out with another woman, who was tired of his "entanglement" with Yoo.

Sgt. Richard Anderson, a coworker, testified that Simmons said five days after the slayings, "I am rid of that b---- forever."