Rufus Isaacs, 66 a retired professor of applied mathematics at Johns Hopkins University, died of cancer Jan. 18 at Johns Hopkins Hospital in Baltimore.

Dr. Isaacs, who was born in New York, received his undergraduate degree from the Massachusetts Institute of Technology. He earned his masters and doctoral degrees from Columbia University. He then joined the Rand Corporation in Santa Monica, Calif. as a mathematician.

He came to Washington in 1959 and joined the staff of the Institute for Defense Analysis and the Center for Naval Analysis. He worked there until 1970, when he became a professor of applied mathematics at Johns Hopkins. He retired in 1977 and was appointed professor emeritus. He was a resident of Baltimore at the time of his death.

Dr. Isaacs was credited with developing a branch of mathematics known as "differential games," and had written a book on that subject. He had addressed numerous meetings of learned socities in this country and abroad.

In his younger days, Dr. Isaacs studied at the Art Students League in New York City. Recently, he had taken classes in sculpture at the Maryland Institute of Art.

Survivors include his wife, the former Rose Bicov of Baltimore, and two daughters, Ellen, of New York City, and Frances, of Washington.