Catherine Elizabeth English, 16, of Kensington and Robert W. Demarest, 17, of Washington were killed Tuesday when the car in which they were riding was involved in a head-on collision with a tractor-trailer on U.S. Rte. 50 near Vienna in Dorchester County, Md., Maryland State Police reported.

The driver of the car in which Miss English and Mr. Demarest were riding, identified as Robert Smith, 18, of Kensington, also was killed in the crash.

Police said their vehicle ran off the right side of the highway, struck a pole, swerved back onto the roadway, crossed the center line and collided with a truck. The truck driver was not injured, but a passenger riding with him was treated for injuries and released. Police said no charges had been filed.

Miss English was born in Washington and graduated from Holy Redeemer Elementary School in Kensington. She recently completed her junior year at Immaculata High School in Washington and was a member of the school honors program. From 1971 to 1974, she lived in Italy while her father was stationed in Naples with the Navy.

In recent months, she had been a theater aide at the Smithsonian Institution's Air and Space Museum.

Mr. Demarest was born in Parma, Ohio. He moved to Washington two years ago. He had completed his junior year at St. John's College High School, where he played on the varsity football team and was president-elect of the Student Council and an officer in the Cadet Corps. He was a member of Blessed Sacrament Catholic Church.

Miss English's survivors include her parents, Rose Marie and Joseph M. English III, a sister, Teresa, and four brothers, Michael, Stephen, Nicholas and Joseph IV, all of Kensington, and her grandmothers, Dorothea English of Silver Spring and Elizabeth Delaney of Washington.

Mr. Demarest's survivors include his parents, Helen M. and George W. Demarest Jr., a sister, Gail, and two brothers, Geoffrey and Brian, all of Washington, and a grandfather, George W. Demarest Sr. of Buffalo.