Fifteen months ago, Marx Roscoe (Moxie) Jackson got into a late-night quarrel in front of a Southwest Washington restaurant and pointed a pistol at the chest of Eddie Jerome Mathis.

"I'm going to blow your head off," Jackson said to Mathis, according to prosecutors who recounted the incident in court. "If you're going to kill me," Mathis retorted, "Do it now."

Jackson backed off, said prosecutors, who described both men as drug dealers. Three weeks later, on Sept. 25, 1981, Moxie Jackson lay dead, slumped over the steering wheel of his late-model car at Ninth and U streets NW, four bullets in his head.

For the last two weeks, in a trial marked by extraordinary security precautions, a D.C. Superior Court jury has listened to witnesses describe the incident at the restaurant and the subsequent shooting. Yesterday, the jurors found Eddie Mathis guilty of first-degree murder in Jackson's death.

The jury found two other men -- Mathis' brother Larry and Harry A. Jackson -- not guilty.

Assistant U.S. Attorney Mark Biros told the jurors the motive for the murder was "respect" as much as anything else. Moxie Jackson's threat, made in front of the Gangplank Restaurant, was seen by four women, he said.

Describing Eddie Mathis as the "self-proclaimed Godfather" of the city's drug trade, Biros told the jury that in the code of the city's drug underworld Mathis could not let the threat go unanswered.

Eddie Mathis' lawyer, R. Kenneth Mundy, argued that the prosecution's version of what happened at the restaurant was "inconceivable" and that witnesses to it were unreliable.

Mundy and defense lawyers Charles Stowe and Alan Dale focused much of their argument on the government's key witness, Deborah Jones, Mathis' second cousin, who testified that she participated in the shooting of Moxie Jackson.

The defense lawyers argued that the government allowed Jones to plead guilty to a lesser charge of accessory to murder and dropped several other criminal charges against her in return for her testimony against the men. The prosecution's case, Mundy said, hinged on the "fertile imagination of Deborah Jones."

In addition to Jones, two government witnesses identified Eddie Mathis as one of the gunmen, but neither witness identified Harry Jackson, charged with being the other gunman, or Larry Mathis, charged with driving the getaway car.

Law enforcement officials, who have described Eddie Mathis as one of the city's major drug traffickers, said that before the trial threats had been made against Biros and Jones, who is now in the federal witness security program.

During the trial, everyone entering the courtroom was checked for weapons. About a dozen U.S. marshals stood guard throughout. A bulletproof glass partition separated the participants from the spectators. Marshals accompanied Biros wherever he went in the courthouse and at his home.

Four years ago, another federal prosecutor investigating heroin trafficking was shot and wounded slightly as he was walking into the federal courthouse here. No one has been prosecuted in that shooting.

The jury deliberated nearly three days before reaching its decision. After the verdicts were read, Eddie Mathis smiled and waved to family and friends in the courtroom as he was led away by marshals. Larry Mathis was released and Harry Jackson was returned to prison, where he is serving a five-year sentence on a narcotics conviction.

Judge Fred B. Ugast said he would pronounce sentence Jan. 7 on Mathis, who faces a minimum of 20 years and a maximum of life in prison.