BOSTON >> Stephen “The Rifleman” Flemmi, the former partner of reputed gangster James “Whitey” Bulger, testified today that he watched as Bulger strangled Flemmi’s stepdaughter, a woman who had called him “Daddy” since she was a toddler.

Flemmi, testifying during Bulger’s racketeering trial, said Bulger killed Deborah Hussey because she was using drugs, getting arrested and dropping their names when she got in trouble.

Under cross-examination, Flemmi denied a suggestion from Bulger’s lawyer that he, not Bulger, killed Hussey. But Flemmi also acknowledged that he twice had oral sex with Hussey in what he called “a moment of weakness.”

Bulger, 83, is accused of participating in 19 killings during the 1970s and ’80s while leading the notorious Winter Hill Gang. He fled Boston in 1994 and was one of the nation’s most-wanted fugitives until he was captured in Santa Monica, Calif., two years ago.

Flemmi, 79, pleaded guilty to 10 murders and is serving a life sentence. Today was his third day on the stand, and his cross-examination began near the end of the day’s court session.

When Bulger attorney Hank Brennan initially asked Flemmi about whether he had a sexual relationship with Hussey, Flemmi asked him to clarify whether he was talking about intercourse or oral sex. He then acknowledged having oral sex with her twice.

“I never inflicted any abuse on her; that was consensual,” Flemmi said, prompting murmurs from family members of some of Bulger’s alleged victims in the courtroom.

Flemmi said Hussey’s mother, Marion, had been his live-in girlfriend since Hussey was just a toddler and that she had called him “Daddy” from a young age.

“A girl who called you ‘Daddy’ consented to sex with you?” Brennan asked.

Flemmi said the sexual contact took place when Hussey became “a different person” because of “her demeanor, her lifestyle.” He said she had been frequenting Boston’s Combat Zone, which was then a center of drugs and prostitution.

“So that gave you an invitation to have sexual relations with her?” Brennan asked.

“On two occasions, and I regret it, in a moment of weakness,” Flemmi said.

Neither Brennan nor Flemmi specified how old Hussey was at the time. She was 26 when she was killed in 1985.

Flemmi said Hussey was killed “at the insistence of Jim Bulger.”

“Mr. Brennan, I didn’t strangle her,” he said.

Flemmi said he reluctantly agreed to bring Hussey to a home in South Boston.

“Jim Bulger stepped out from behind the top of the basement stairs and grabbed her by the throat and started strangling her,” Flemmi said. “He lost his balance and they both fell on the floor, and he continued strangling her.”

Flemmi said he and Bulger henchman Kevin Weeks carried her body downstairs in the basement, where she was buried in the dirt floor.

He said Bulger went upstairs to lie down after he killed Hussey.

Hussey was one of three people who were buried in the same house after Bulger killed them, Weeks testified previously. He said he helped moved the bodies later when the house was being sold. In 2000, after he began cooperating with the government, he led authorities to the bodies.

Earlier in the day, the brother of another 26-year-old woman who was allegedly killed by Bulger exploded in anger after Flemmi identified him as a drug user and informant.

Steven Davis jumped out of his seat in the courtroom and shouted at Flemmi, “That’s a lie!”

Flemmi had testified Friday about the 1981 killing of Debra Davis, Flemmi’s girlfriend. Flemmi said Bulger strangled her after Flemmi told her that he and Bulger were FBI informants.

Today, when Assistant U.S. Attorney Fred Wyshak asked him to clarify and reminded him that Davis had several brothers, Flemmi corrected himself and said he meant Mickey Davis, not Steven Davis.

“I apologize for that remark,” Flemmi said.