Samuel Bertolet Detwiler Jr., 69, a retired research chemist and administrator who worked for the U.S. Department of Agriculture for almost 30 years, died Sunday at the Sleepy Hollow Nursing Home in Fairfax. He had been ill since suffering a heart attack in 1973.

Born in Wabasha, Minn., Mr. Detwiler's family moved to Virginia when he was a boy and he graduated from the old Western High School in Georgetown. He received his bachelor's degree in chemistry in 1934 from the George Washington University and his master's degree in organic chemistry from the University of Illinois.

After working as a research chemist at the Department of Agriculture's Regional Soybean Industrial Products Laboratory in Urbana, Ill., for three years, he moved to Washington in 1944.

In Washington Mr. Detwiler served as project officer of the Bureau of Agricultural and Industrial Chemistry and later became assistant to the administrator of the Agricultural Research Service. He held this position for 14 years, retiring in 1972.

While employed with USDA in Urbana, Mr. Detwiler assisted in research that resulted in the soybean being used as the primary source of vegetable oil for shortenings and margarine in the United States. While in Washington, he was involved in directing agricultural research in foreign universities.

After retiring from USDA, Mr. Detwiler served as research associate with the Federation of American Societies for Experimental Biology in Bethesda.

He received the Honor Scroll of the American Institute of Chemists and the Public Service Award of the Chemical Society of Washington. He was a Fellow of the Washington Academy of Sciences and was editor of its journal.

Other memberships included the Chemical Society of Washington, the American Institute of Chemists and the American Oil Chemists's Society.

Survivors include his wife, Kathryn N., of the home in Arlington.

The family suggests that expressions of sympathy be in the form of contributions to the American Heart Association or to a charity of one's choice.