Robert W. (Bob) Johnston, 57, a government liaison officer with the Potomac Electric Power Co., was fatally injured Friday in an automobile accident on U.S. Rte. 301 near La Plata, Md.

Charles County sheriff's officers said Mr. Johnston, who lived in Silver Spring, was driving south on 301 when his car was struck by another vehicle at a crossroads. Sheriff's officers said the driver of the second car was charged with failure to yield the right of way.

Mr. Johnson was pronounced dead at Southern Maryland Hospital in Clinton.His wife, Carolyn, was hospitalized there with injuries. The couple were headed for their vacation home at Ophelia, Va., when the accident occurred.

Mr. Johnson was born in Keyser, W. Va. He grew up there and in Mount Rainier and in Silver Spring. He graduated from McKinley Technical High School in Washington.

He began his career with Pepco in 1941 in the meter department. After World War II service in the Army Air Corps in Europe, he returned to Pepco and worked his way up to liaison officer with the federal government.

He was a former member of the Mount Rainier United Methodist Church and a past commander of American Legion Post No. 110 in Mount Rainier. He also was a past master of the West Gate Masonic Lodge No. 220.

In addition to his wife, of Silver Spring, survivors include three children, Bruce and Andrew, both of Silver Spring, and Joan, of Wheaton; two brothers, Fred R. Johnston Jr., of South Bethany Beach, Del., and Richard S. Johnston, of Houston, Tex.

The family suggests that expressions of sympathy be in the form of contributions to the Waldorf Rescue Squad, Waldorf, Md.