Charles T. Mertz, 55, a retired Foreign Service officer of the U.S. Information Agency, died Friday in Selinsgrove, Pa., after apparently suffering a heart attack.

Mr. Mertz, who had worked in many parts of the world during 29 years of federal service, had lived in Mifflinburg, Pa., since his retirement from the USIA here in 1971. At the time of his death, he was executive director of the Union County Redevelopment and Housing Authority.

Born in Lewisburg, Pa., he served with the Army Air Forces in World War II. He then worked as a civilian in the civil affairs division of the Army, producing films for reorientation programs for people in occupied areas of Germany, Austria, Japan and Korea.

After the program shifted to the State Department, he became regional director of motion pictures for the Far East. He later established an information program for the Republic of Vietnam, then spent seven years on assignment in Brazil before joining the USIA office in New York in 1964.

Mr. Mertz moved later to the Washington office of USIA. He was in charge of liaison with commercial newsreel companies and produced a series of screen magazines for distribution in South America and Africa.

He is survived by his wife, Catherine Finck Mertz, and a daughter, Barbara Kay, of the home; two sons, Kenneth B., of Princeton, N.J., and Ronald A., of Daneville Pa.; his father, George M. Mertz, three brothers, and a grandchild.