James Williams Riddleberger, 78, an authority on central European affairs who retired from the State Department in 1967 as a career ambassador, the highest rank in the U.S. Foreign Service, died of a heart attack Oct. 16 at the Shenandoah County Memorial Hospital in Woodstock, Va. He lived in Woodstock.

Mr. Riddleberger served as U.S. ambassador to Yugoslavia, Greece and Austria. In the late 1940s, when the Cold War began, he played important policy roles in Germany and in the development and administration of the Marshall Plan, which revived the economy of Europe after World War II.

In later years, he helped mediate disputes between Yugoslavia and Italy over Trieste and between Greece and Turkey over Cyprus. In 1959, he headed the International Cooperation Administration, a predecessor to the Agency for International Development. He was ambassador to Austria when he retired.

Mr. Riddleberger was born in Washington. He graduated from Randolph-Macon College and earned a master's degree in international relations at Georgetown University. He worked for the Library of Congress and the Tariff Commission before joining the Foreign Service in 1929.

His first foreign posts were Geneva and Berlin. During the war, he was chief of the Central European Affairs division at State.

After the war, he returned to Berlin as counselor to the U.S. mission there. He became the chief political adviser to Gen. Lucius D. Clay, the U.S. commander who ordered the Berlin airlift into operation, and later to John J. McCloy, Clay's successor as U.S. high commissioner in occupied Germany.

In 1950, Mr. Riddleberger was transfered to Paris, where he had responsibilities in the U.S. embassy and also in the administration of the Marshall Plan, where he was senior political advisor.

After a tour in Washington as director of the bureau of German affairs with the rank of assistant secretary of State, Mr. Riddleberger began a four-year stint as ambassador to Yugoslavia in 1953. He was in that post when Yugoslavia and Italy settled their differences over the Adriatic port of Trieste.

He also was credited with helping to persuade Marshal Josip Broz Tito, the Yugoslav leader, to resist blandishments from Nikita S. Khrushchev to return to the Soviet orbit, from which Yugoslavia had broken away in 1948.

From 1957 to 1959, Mr. Riddleberger was ambassador to Greece, where he tried to smooth relations between that country and Turkey over Cyprus. He then became director of the ICA. In 1961, he went to Paris as chairman of the development assistance committee of the Organization for Economic Cooperation and Development. He went to Austria as ambassador in 1962.

After his retirement, Mr. Riddleberger moved to his family home in Woodstock. He served as a president of DACOR (Diplomatic and Consular Officers Retired) and as honorary chairman of the Population Crisis Committee.

He was a member of the University and Metropolitan clubs.

Survivors include his wife, Amalie, of Woodstock; two sons, Christopher, of Far Hills, N.J., and Peter, of Washington; a daughter, Antonia Stearns of Athens, Greece; a brother, Patrick, of Edwardsville, Ill.; a sister, Virginia Kennedy of Reston, and nine grandchildren.