A District cabdriver was shot in the back of the head and killed early today, apparently during a robbery, as he drove his taxi in Southwest Washington, D.C. police reported.

The driver of Dial Cab Co. cab No. 546, whose name was not immediately available, was found shortly after midnight slumped in his vehicle where it rolled to a stop on a traffic median at Sixth and M Streets SW, police said. He was pronounced dead at George Washington University Hospital at 12:38 a.m.

Police said the driver was found by officers after a resident of the area, near Arena Stage, reported hearing gunshots and flagged down a passing police car.

A Dial dispatcher said early today that the taxicab was not equipped with a radio and that records that would identify the driver were locked in the company's office.

Homicide detective Daniel Villars said it was not known how many passengers were in the vehicle when the shooting occurred or where they had been picked up. He and other investigators were studying the driver's trip sheet.

The last D.C. cabdriver to be killed was Douglas R. Thornton, 42, who was struck in the head with a blunt instrument and left to die in an Arlington parking lot in January.

Last Nov. 1, a graduate student in clinical psychology who was moonlighting as a cabdriver was killed in Northeast Washington when he was struck on the head, shot in the neck and thrown out of the cab, which was later burned.