Paul H. Johnstone, 78, an authority on agricultural history and a retired employe of the Institute for Defense Analyses in Arlington, died Friday at Northern Virginia Doctors Hospital after a stroke. He lived in Fairfax.

From 1955 until his retirement in 1961, Mr. Johnstone was a project leader for the Institute, a research organization for the Defense Department.

A former educator and the author of publications on agricultural history, he joined the government in 1936 as a speechwriter in the Office of the Secretary of Agriculture.

He began his career in intelligence during World War II, when he was a division director for the old Board of Economic Warfare. He also spent a year in India and Ceylon as a member of the staff of Britain's Adm. Lord Mountbatten, when he was directing Allied operations in Southeast Asia against Japan during the war.

After the war, Mr. Johnstone was a regional economist in California for the Interior Department's old Bureau of Reclamation and an economic adviser in China for the old Economic cooperation Administration.

From 1949 to 1955, when he joined the Institute for Defense Analyses, he was a section chief with the Air Force Intelligence Service.

Mr. Johnstone was born in Monte Vista, Colo. He grew up in Minnesota and earned bachelor's, master's and doctor's degrees from the University of Minnesota. He taught history and French at various colleges before beginning his government career.

Mr. Johnstone was a member of the board of directors of the American Rose Society. He held several offices with the Arlington Rose Foundation and the Potomac Rose Society.

Survivors include his wife, Polly, of Fairfax; a daughter, Diana, of Paris; a son, Paul Jr., of Indianapolis; a brother, Bruce, of Minneapolis, and three grandchildren.