A teenage bicyclist and the operator of a motorcycle were killed yesterday in separate collisions with automobiles in the Maryland suburbs, authorities said. In the motorcycle accident, which occurred in Prince George's County, the driver of the car fled on foot, county police said.

Police were searching for the driver late yesterday near the scene of the crash, which occurred about 4 p.m. at Woodyard and Dower House roads, just east of Andrews Air Force Base, police said.

The driver of the motorcycle, Thomas J. Persell of Lothian, was pronounced dead at the crash site, said Cpl. Kim Brown, a police spokeswoman.

Persell was southbound on Woodyard when he was struck head-on by a 1991 Honda Accord that was attempting to turn left onto Dower House, Brown said.

Police said they were trying to determine whether speed played a part in the collision.

A firefighter who went to the scene said Persell wore a helmet and appeared to be in his twenties. He was thrown about 150 feet, the firefighter said. According to the firefighter, the Accord was demolished.

Brown said the driver climbed out and left the scene. Residents of the area, who noted that Woodyard has become increasingly congested in recent years, said the road was shut down for hours.

The bicycle collision occurred about 5:45 p.m. in Gaithersburg.

Police said a 15-year-old was riding a bike when he was struck by a westbound Volkswagen Jetta on Great Seneca Highway. Police said they were withholding the name of the teenager until relatives could be notified.

Police said the bicyclist was attempting to cross Great Seneca. They said they had not determined whether the teenager was in or near the crosswalk when he was struck, or what the sequence of traffic lights was at the time he tried to cross the intersection.

Staff writer Ann E. Marimow contributed to this report.