Jerome St. Julien Ducrest, 73, a retired trial attorney for the Department of Agriculture who formerly taught political science, died Tuesday at George Washington University Hospital. He had leukemia.

Mr. Ducrest moved to this area in about 1930 and was a political science professor at Georgetown University before joining the Agriculture Department in 1933 as a trial attorney. He worked in Agriculture's old packers and stockyards administration, now part of the Agricultural Marketing Service, until retiring in 1972.

He was born in Broussard, La., and graduated from Louisiana State University in Baton Rouge. He earned law degrees from the law schools at Georgetown University and Tulane University in New Orleans.

During World War II, Mr. Ducrest served in the Army Air Corps' meteorology division.

Survivors include his wife, Rosina B., a daughter, Antoinette Marie, and two sons, Jerome L., and Edouard M., all of Arlington; four sisters, Marcella Lewis, Sylvia Shipp and Irene Updike, all of Broussard, and Leona Melson of Columbia, S.C., and three brothers, Thomas L. and Francis M., both of Broussard, and James R., of Hampton, Va.

The family suggests that expressions of sympathy be in the form of contributions to the Virginia Division of the American Cancer Society.