Melvin Douglas Downing, who was convicted of three 1976 murders at a Northeast Washington fish market but later was acquitted in these cases, was found guilty yesterday by a D.C. Superior Court jury of a slaying committed last November in the course of an apparent street robbery.

Downing, 25, who was on trial with Walter Ricardo Jefferson, 18, was charged with first-degree murder in the death of 48-year-old Walter (The Count) Robinson last Nov. 25 near the latter's home in the 1200 block of New Jersey Avenue NW.

Robinson, who at the time was walking with his 18-year-old godson, Victor Solomon, was shot to death during an apparent robbery attempt.

Witnesses in the trial identified Downing as the man who drove the car that sped away from the scene carrying several other persons after Robinson had been shot and Solomon robbed of $14.

Assistant U.S. Attorney Reggie Walton told the jury that Downing and several other men had been using drugs on the day of the murder. When they ran out of drugs, Walton said, Downing had driven to the New Jersey Avenue area with two other men in search of drug dealers to rob."

Walton contended in the trial before Judge George H. Revercomb that after the men noticed Robinson and Solomon, Herman (Popeye) Williams -- the third man charged in the slaying who appeared as a government witness -- got out of the car and demanded money.

The government contended that Downing handed a revolver to Williams and Williams approached Robinson, who told Williams, "Back up, you young punk. I ain't giving you nothing."

Then Jefferson left the car, took the revolver from Williams, said, "Nobody's gonna buck on me" and fired three times, striking Robinson in the back and in the arm, witnesses testified.

Downing was charged on Feb. 11, 1976, with the execution-style slayings of three men in the Berkeley Farms Poultry and Fish Market at 1348 Florida Ave. NE.

After a two-week trail, Downing was convicted of the felony murder in the deaths of two employes of the market and a customer. His attorney, R. Kenneth Mundy, requested a new trial after it was learned that one juror who had heard the case visited the fish market during deliberations before he entered his vote of guilty.

Downing testified during his retrial that he had been at the fish market earlier on the night of the murders, but had left before the slayings occurred.

O. B. Parker, Downing's attorney in the murder trial this week, did not allow his client to testify. But Parker aruged in his closing statement that the government had not proved its case against Downing.

Michael Rankin, Jefferson's attorney, told the jury that government witnesses had lied and repeatedly contradicted each other during the case and changed testimony they had given in the past.

Both Downing and Jefferson were found guilty of felony first-degree murder, armed robbery, and attempted armed robbery. Jefferson was also found guilty of carrying a gun without a license.

The men could receive up to life imprisonment for the murder and robbery convictions. Sentencing in the case is scheduled for Nov. 1.