Sergius O. Yakobson, 78, a retired Library of Congress senior Russian affairs specialist and Legislative Reference Service division cheif, died Tuesday at George Washington University Hospital following a series of strokes.

Dr. Yakobson joined the Library of Congress in 1941 on a fellowship from the Rockefeller Foundation. He was promoted from Slavic history consultant to foreign affairs analyst in the library's Legislative Reference Service in 1947.

He was named cheif of the foreign affairs division of the LRS in 1949, and a year later became the senior specialist in Russian affairs for the LRS. In 1951, the Slavic and Central European Division of the library was established. Dr. Yakobson was named its director and held the post until retiring in 1971. He then served as the library's honorary consultant in Slavic studies until his death.

He represented the Library of Congress at the Second International Congress on Southeast European Studies, which was held in Athens, Greece, in 1970.

He was a past chairman of the Washington chapter of the American Association for the Advancement of Slavic studies and received the organization's 1972 National Award for his distinguished contributions to the field of Slavic studies.

Dr. Yakobson was chairman of the Conference on Slavic and East European History of the American Historical Association in 1964.

He had contributed articles to the Annals of the American Academy of Political and Social Science, the Journal of International Law, and the Harvard Slavonic Papers. He also was the author of government reports.

Dr. Yakobson was born in Moscow. He was attending Moscow University when the Russian Revolution forced his family to flee. He earned a doctoral degree in history at the University of Berlin and then worked as a research fellow in the Prussian State Archives. He was fired in 1933 because he was a Jew.

He went to England where, with the help of Arnold Toynbee, he obtained a lectureship at King's College, the University of London. He also taught at Oxford and Cambridge universities and wrote the Encyclopedia Britannica. a

During World War II he translated Russian radio broadcasts for the British Ministry of Information before coming to this country.

He is survived by his wife, Helen L., of the home in Washington; a son, Denns L., of Denver; a daughter, Natalie A. Reatig of Washington; a brother, Roman, O., of Cambridge Mass., and three grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the S. Yakobson Memorial Fund in Russian studies at the School of Public and International Affairs at George Washington University.