William L. McGovern, 64, one of the country's leading antitrust trial lawyers, died Sunday at his home in Bethesda. He had been in ill health since suffering a stroke in 1974.

Mr. McGovern retired last year as a partner in the law firm of Arnold & Porter. He had joined the firm in 1951.

Born in Bridgeport, Conn., he graduated from Yale University and its law school. He joined the antitrust division of the Justice Department in 1938.

He was one of the government trial lawyers in its suit against the Pullman Co. which led to the divestitute of the company from the railroads.

During World War II, Mr. McGovern served as a civilian with the Bureau of Economic Warfare of the Foreign Economic Administration in China and was awarded the Medal of Freedom.

During his years with Arnold & Porter, he represented defendants in a number of major antitrust cases and also lectured and wrote on antitrust subjects.

He is survived by his wife, Hermonine McGovern, of the home; two son, William Jr., of Starkville, Miss., and John, of Hillsboro, N.C.; two brothers, Thomas and John, of Bridgeport; a sister, Katherine Flannigan, of Fairfield, Conn, and two grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to Children's Hospital.