Charles W. Yost, 73, a former U.S. ambassador to the United Nations and a career diplomat whose foreign posts took him to the Middle East, Europe and Southeast Asia, died of cancer Thursday at Georgetown University Hospital.

Mr. Yost joined the Foreign Service in 1930 and was sent to Alexandria, Egypt, as a consular officer. His next assignment was Warsaw. After a brief period in private life, he joined the State Department in Washington. He attended the Dumbarton Oaks Conference in 1944, the San Francisco Conference in 1945, where the United Nations was organized, and the Potsdam Conference shortly after the end of World War II in Europe.

In the late 1940s and early 1950s, he served in Czechoslovakia, Austria and Greece at times when these countries were under intense pressure from the Soviet Union or just emerging from it. In 1954, he was named U.S. minister to Laos and then became the first U.S. ambassador to that nation. In 1957, he was the minister counselor, or second-in-command, of the American Embassy in Paris. At the end of that year, he was named U.S. ambassador to Syria. Shortly after his appointment, Syria and Egypt formed the United Arab Republic. Mr. Yost was named ambassador to Morocco in 1958.

In 1961, he began his first assignmewnt at the United Nations as the deputy to Adlai E. Stevenson. Following Stevenson's death in 1965, Mr. Yost stayed on to be deputy to Arthur Goldberg. He resigned from the Foreign Service in 1966 to begin a career as a writer and teacher on foreign affairs.

In 1969, president Richard Nixon nominated Mr. Yost as the permanent U.S. representative to the United Nations, the official title of the U.S. representative there. He resigned in 1971 and returned to writing and teaching. Mr. Yost held the rank of career ambassador, the highest member of the profession Foreign Service can reach.

As his career progressed, Mr. Yost developed a reputation for a balanced and evenhanded approach to international questions. In the tradition of classical diplomacy, he went about his work quietly. His style contrasted sharply with the high public profile kept by such predecessors at the United Nations as Henry Cabot Lodge, Stevenson, Goldberg and his successor, George W. Bush, now the vice president. He believed that if force should replace the political process, it should be used with care and in pursuit of limited objectives. He was a critic of the massive U.S. intervention in Vietnam that began in 1965 under President Johnson, stating that no American interest was in jeopardy that would justify such an effort.

During his years as head of the U.S. mission to the United Nations, Mr. Yost was chairman of a four-power committee -- the other members were Britain, France and the Soviet Union -- whose purpose was to achieve a ceasefire in the smoldering war between Egypt and Israel. At the time of his resignation, some news reports said the Nixon administration had come to doubt that his support of Israel in the face of Arab claims was unqualified. Mr. Yost denied that he had any policy differences with the administration in carrying out his duties.

At the same time, however, he developed an increasingly pessimistic view of the world. He was concerned, among other things, about population, pollution and the environment, the continuing poverty of the Third World, and the unequal distribution of wealth in the developed nations.

Mr. Yost set these views forth in a syndicated newspaper column and in three books that he wrote. The most recent is "History and Memory," which appeared last year. His earlier books were "The Insecurity of Nations" and "The Conduct and Misconduct of Foreign Relations." He also taught at the Columbia University School of International Affairs and the Georgetown University School of Foreign Service.

In 1979, Mr. Yost was cochairman of Americans for SALT, a group that lobbied the Senate for passage of the second Strategic Arms Limitation Treaty. He was trustee of the American University in Cairo, Egypt, and director of the Aspen Institute program for cultural exchanges with Iran. He took part in several unofficial conferences between U.S. and Soviet scholars. In 1973, he was named head of the National Committee on U.S.-China Relations and visited the People's Republic of China in 1973 and 1977.

Charles Woodruff Yost was born in Watertown, N.Y. He was educated at the Hotchkiss School and at Princeton University, from which he graduated in 1928. He spent a year studying at the University of Paris before joining the Foreign Service.

Survivors include his wife, the former Irena Oldakawska, whom he met while serving in Poland, of Washington; three children, Nicholas and Casimir, of Washington, and Felicity, of New York City, and three grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the DACOR Educational and Welfare Foundation, Washington.