A Virginia man apparently drowned in the Potomac River near Mount Vernon yesterday afternoon, and the search continued upriver for a 16-year-old Kensington girl who slipped off the rocks south of Great Falls on Saturday, authorities reported.

The man, whose name was not released pending notification of his family, was swimming off a cabin cruiser about a half-mile north of Mount Vernon and 500 yards from shore about 3:30 p.m. when another boater noticed him struggling and went to his aid, according to U.S. Coast Guard spokesmen.

They said the man was pulled into the other boat, where an unsuccessful effort was made to resuscitate him. He was pronounced dead when the boat reached rescue vehicles waiting on shore, the spokesmen said.

The missing girl, identified as Kelly A. King, was with friends on the rocks near Rock Island when she slipped and fell into the water about 5 p.m. Saturday, Montgomery County police said.

They said the friends attempted to pull her to safety, but that she disappeared beneath the surface.

At least six people have drowned this year in the treacherous 11-mile stretch of the river between Seneca and the Chain Bridge. Last year, 14 people drowned in that stretch.