A Virginia judge yesterday denied a prosecution request for psychological testing of Phillip Steeves, the Marine corporal accused in the May 20 sniper slaying of a young woman motorist on an Alexandria street.

Alexandria chief prosecutor John E. Kloch, arguing for the tests to determine whether 21-year-old Steeves is mentally fit to stand trial, said police who questioned the sevicemen shortly after his arrest found he "didn't understand . . . didn't know what he was doing."

But General District Court Judge Robert T. S. Colby denied the motion, saying Steeves "has been responsive to questioning." Colby sided with Steeves' attorney, Alfred Swersky, who contended his client is "clear, lucid and competent to aid in his own defense."

Kloch, arguing for the request, also said police investigating the slaying of the motorist, Mary Anne Monteclavo, 24, of Arlington, now believe that Steeves may have been involved in a total of six shooting incidents during a 45-minute period instead of three incidents reported earlier. Kloch refused to elaborate on his statement.

A 28-year-old Fairfax County man was seriously wounded while driving on the Woodrow Wilson Bridge and a Stafford County man near the bridge was shot at but not harmed within minutes of the Monteclavo incident. Police have said Steeves is a suspect in the shootings.

Arlington police have said they are investigating a shooting incident in which a car was struck by a bullet in the 1400 block of Jefferson Davis Highway between 1 and 1:30 a.m. on May 20, but have made no formal connection with Steeves.

A preliminary hearing for Steeves is scheduled for June 11.