Gregory Benson was acqitted yesterday of a charge of killing his Capitol Hill neighbor, Frank Flook, in a widely publicized shooting during Christmas week of 1979.

Benson, 27, had been tried last October on the charge. But D.C. Superior Court Chief Judge H. Carl Moultrie I declared a mistrial after the jury could not reach a verdict. Benson has consistently maintained his innocence in the case.

He stood silently as the jury declared him innocent of first-degree murder and two burglary charges. Flook's widow, Marilyn, sitting in the courtroom, sobbed quietly as members of Benson's family grabbed one another and swayed and cried. The judge shouted down the demonstration.

"Mr. Gregory Benson," Judge Moultrie said, "You have been found 'not guilty' by a jury of your peers.. All charges in this case are no longer standing against you."

Benson, however, was not freed because he is serving a 7-to-20-year prison term for unrelated burglary charges and will not be eligible for parole for seven years, according to his lawyer. That case is on appeal.

The 33-year-old Flook was shot in the head as he and his wife tried to prevent two men from forcing their way into the Flook's Capitol Hill row house at 1521 A. St. NE the evening of Dec. 27, 1979.

Flook, assistant manager of a suburban Maryland garden center, and his wife, an employee of the Department of Housing and Urban Development, had moved into their newly renovated town house just three months earlier. The Flooks were the only white couple living in the block of row houses, and the shooting outraged friends and neighbors, both black and white, who offered a $1,000 reward for information leading to the arrest of the killer.

According to police reports, as the intruders attempted to push open the door, one of the intruders reached in with a pistol and shot Flook.

The two men then fled. Benson was picked out of a police lineup by Marilyn Flook about three months afte the incident as one of the two men involved.

Benson and witnesses who appeared in his behalf asserted that Benson was in his own home at the time.

"The facts of this case spelled out two words -- 'not guilty,'" Benson's attorney, Timothy D. Junkin, said after the verdict.

"I'm so glad it's over, said Benson's father, Joseph M. Benson, who had said that he saw his son at home at the time of the incident.

Assistant U.S. Attorney Reggie B. Walton, the prosecuter in the seven-day trial, said he felt "the evidence supported the government, but the jury felt otherwise."

Marilyn Flook, who testified during each of the trials about the night her husband died, left the courthouse in tears. "I feel like a victim of the two men on the porch, and I also feel like a victim of the system," she said later. "I'm very disappointed. It's very hard all around.

Flook said that she hoped that the killer eventually would be apprehended.

In another development, two government witnesses in the case -- Curtis Elmore and Raymond S. McCray -- who contend that police forcibly obtained statements from them, have filed a $550,000 lawsuit against the D.C. police department. Their allegations are being investigated by the police department's internal affairs division.