William Joseph Sebald, 78, a former Naval intelligence officer and lawyer who served as a political adviser to Gen. Douglas MacArthur in Japan and later as ambassador to Burma and Australia, died Aug. 10 at the Naples (Fla.) Community Hospital. He had emphysema.

Mr. Sebald was serving as counselor to George C. Atcheson Jr., chairman of the Allied Council for Japan, in 1947 when Mr. Atcheson was killed in a mid-Pacific airplane crash.

Gen. MacArthur then appointed him to succeed Mr. Atcheson as chief of the Allied Headquarters diplomatic section and chairman of the Allied Council for Japan, with the personal rank of ambassador.

During this time, Mr. Sebald played a leading role in the repatriation of more than 500,000 Japanese prisoners of war from Siberia and Soviet-controlled areas. He also worked on the formulation of the Japanese peace treaty. He was named U.S. political adviser for Japan in 1950.

Shortly before the treaty became effective in early 1952, he was appointed U.S. ambassador to Burma. He served in this capacity until 1954, when he became deputy assistant secretary for far eastern affairs in the State Department here.

He then served as U.S. ambassador to Australia from 1957 until retiring in 1961.

A native of Baltimore, Mr. Sebald graduated from the Naval Academy at Annapolis in 1922. In 1925, he became Naval attache at the American Embassy in Tokyo as a language officer.

After resigning from the Navy in 1930, he earned a juris doctor degree from the University of Maryland's law school.

A specialist on international law, he practiced law in Japan from 1933 to 1939. He wrote a number of books on Japanese law, including original translations of Japanese civil, criminal and commercial codes. In 1949, he received an honorary doctor of laws degree from the University of Maryland for his work in Japanese law.

He also had a private law practice here for about two years until the Japanese attack on Pearl Harbor, when he rejoined the Navy as a lieutenant commander. He was assigned to the office of naval intelligence and served as chief of the Pacific section of the combat intelligence unit of the First Fleet. He was awarded the Legion of Merit for this work.

Mr. Sebald lived in Washington until 1966, when he and his wife of 53 years, the former Edith Frances deBecker, moved to Naples.

He was the author of "With MacArthur in Japan -- A Personal History of the Occupation of Japan," published in 1965. He was the coauthor with Dr. C. Nelson Spinks of "Japan: Prospects, Options and Opportunities," published in 1966.

Mr. Sebald was awarded the First Class Order of the Rising Sun with Grand Cordon by the emperor of Japan. He was a life member of the U.S. Naval Academy Alumni Association and a former president of the Asiatic Society of Japan and the Japan-America Society of Washington.

He is survived by his wife.