James “Whitey” Bulger and Stephen “The Rifleman” Flemmi exchanged heated words today after Flemmi took the stand in US District Court and described Bulger as an FBI informant who gave information to law enforcement “hundreds of times’’ during their shared criminal careers.

Flemmi testified as a prosecution witness against his one-time partner in the Winter Hill Gang, an appearance that lasted for about 15 minutes before the trial ended for the day. But as the jury was leaving, Flemmi stood in the witness stand with his hands on his hips and could be seen mouthing an expletive at Bulger.

The 83-year-old South Boston gangster replied, though it was not known what he said.

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The two men, who once mapped out extortion and murders together, glared at each other.

“Really,” Flemmi said before being led out of the courtroom.

Bulger is charged in a sweeping federal racketeering indictment with participating in 19 murders in the 1970s and 1980s, extortion, money laundering, and stockpiling guns. He has pleaded not guilty to all charges and is being held without bail. His notoriety grew when he went on the lam for 16 years, evading an international manhunt, and when his cozy ties to corrupt FBI agents were revealed.

Shortly before 1 p.m., Flemmi was sworn in, giving jurors their first look at the 79-year-old who has hearing loss in his right ear. Flemmi was wearing a khaki prison uniform and windbreaker.

“Do you know a man named James Bulger?’’ Flemmi was asked.

“Yes, he’s right there at the table,’’ he replied, describing Bulger’s clothing to confirm the identification.

Flemmi said he last saw Bulger one week prior to Christmas in 1994.

Asked what his relationship with Bulger was, he replied, “Strictly criminal.”

Bulger, through his attorneys, has insisted he was not an informant for the FBI.

Flemmi today was asked directly: Was Bulger an informant for the FBI?

“Yes,’’ Flemmi responded, prompting Bulger to raise his head and look at his former ally.

Flemmi testified that he was on hand “hundreds of times’’ when Bulger provided the FBI with information about other criminals, especially members of the Mafia.

“I was giving him information, and he was giving it to the FBI,’’ Flemmi said.

He said Bulger gave information to corrupt FBI agent John Connolly and other FBI agents.

Flemmi testified about 15 minutes, until 1 p.m., the time the trial ends each day. He is due back on the stand Friday.

Flemmi, according to long-time Boston defense attorney Martin G. Weinberg, “is arguably the most critical witness in the case. He is the single witness who can either adopt or contradict Bulger’s claims that he was not guilty of certain murders and that he was not an informant.”

In letters to a friend from jail last year that were shared with the Globe, Bulger said the two things he most wants to refute are that he was an informant and that he killed two women, who are among his 19 alleged victims. Flemmi is a key government witness on those points.

Flemmi is serving a life sentence for 10 murders and has implicated Bulger in all of them. He claims he watched as Bulger strangled Debra Davis and Deborah Hussey on separate occasions in the 1980s. He is also expected to offer jurors a firsthand account of the corrupt relationship he and Bulger shared with FBI agents and of leaked information he says prompted them to kill other informants.

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