A 5-month-old Charles County girl died early Saturday when the car in which she was riding overturned near Port Tobacco, county sheriff officials said.

Lt. David Gladwell said the sheriff's report showed "there was no seat belt or child seat in use" by the infant when the car driven by her mother, Edna Louise Matthews, ran off Poor House Road in rural western Charles and landed on its side. Matthews' baby, Tunisia L. Travers, was thrown from the car, according to the police.

Some 900 children age 5 or younger died last year in automobile accidents, according to the National Highway Traffic Safety Administration. Most were unrestrained by seats or belts. The Maryland General Assembly passed a law this year requiring motorists to use federally approved child-restraint seats for children younger than 3. However, the law doesn't take effect until Jan. 1.

A state police helicopter took Tunisia to Washington Hospital Center, where she was pronounced dead at 4:30 a.m., 1 1/2 hours after the accident. State police also delivered Matthews, 33, of Pisgah, to Prince George's General Hospital, where she was listed yesterday in fair condition with numerous cuts and bruises.

Matthews' 13-year-old daughter, Melissa, was taken to Physicians Memorial Hospital in La Plata where she was in good condition, a hospital spokeswoman said. Matthews' 15-year-old son, Carlton was there also, in good condition.

The sheriff's department is continuing its investigation, spokesman Sgt. Charles J. McDevitt said yesterday.

The car "ran off the road, overturned and ejected the infant," McDevitt said. "The vehicle then landed on its side, on top of Tunisia ." He said police were unsure what had caused the car to leave the road.

Nellie Matthews, Edna Matthews' mother, said Edna was taking her 13-year-old daughter to a friend's house in Doncaster so that the teen-agers could make an early start with a church group bound for King's Dominion amusement park in Virginia.

Nellie Matthews said her daughter's 1979 Ford Mustang had a child safety seat in it, but said she wasn't sure whether the baby was strapped in.

"The way that car wrecked, it wouldn't have done any good," she said. "Everything in the interior was thrown out."

For 12 years, Edna Matthews has worked for Melwood Farms Training Center, driving a bus for retarded adults.

Staff members at Melwood were "very, very upset" when they heard about the accident, said Barbara A. Woodman, an administrator. "When she comes out, there will be a lot of support to help her get over this," she added.

Woodman said that she spoke with Edna Matthews after the accident and Matthews told her a tire had blown out.