Ralph Earle II, an arms control expert who served during the Carter administration as lead U.S. negotiator at the SALT II talks with the Soviet Union and as director of the U.S. Arms Control and Disarmament Agency, died Jan. 13 at a hospital in Sarasota, Fla. He was 91.

The cause was complications from Parkinson’s disease, his son Ralph Earle III said.

Mr. Earle’s government service spanned three decades, beginning with his appointment in 1968 as a Pentagon aide for international security affairs. A decade later, President Jimmy Carter appointed him to succeed Paul C. Warnke as chief negotiator for the second round of the Strategic Arms Limitation Talks, known as SALT II. Mr. Earle, whose post accorded him ambassadorial rank, had previously acted as Warnke’s deputy.

The SALT II agreement, which would have limited the number of nuclear warheads the two nations could maintain, ultimately broke down after the Soviet Union invaded Afghanistan in 1979.

Mr. Earle, whom the New York Times described as “an able administrator who has managed to steer clear of controversies,” went on to lead the Arms Control and Disarmament Agency from 1980 until Carter was succeeded by President Ronald Reagan in 1981.

Having begun his career in legal practice, Mr. Earle worked with the Lawyers Alliance for Nuclear Arms Control, later the Lawyers Alliance for World Security, before returning to the arms control agency as deputy director in 1994. He retired in 1999.

Once asked whether he felt burdened by his work in arms control, or by the potential consequences if diplomacy gave way to war, he remarked that “you think about it but can’t dwell on it,” according to the Times.

Ralph Earle II was born in Bryn Mawr, Pa., on Sept. 26, 1928. His father, George H. Earle III, was a Democrat who served as Pennsylvania governor from 1935 to 1939.

Mr. Earle accompanied his father on diplomatic postings as U.S. minister to Austria in 1933 and 1934, during the run-up to World War II, and as U.S. minister to Bulgaria from 1940 until the country declared war on the United States in 1941.

Early that year, George Earle touched off what the Times described as a “cafe skirmish” in the Bulgarian capital by requesting that a band play “Tipperary,” a British marching song from World War I. An enraged German officer silenced the Bulgarian band, which shifted to a German waltz. Bottles began flying through the air, sending guests scurrying under tables, and making news across the Atlantic.

The younger Mr. Earle received a bachelor’s degree in history in 1950 from Harvard University, then served in the Army before completing studies at Harvard Law School in 1955.

He practiced law in Philadelphia with the firm now known as Morgan Lewis before joining the Defense Department. His assignments included advising the U.S. mission to NATO.

Mr. Earle was a Washington resident before settling in Florida last year. He belonged to the Metropolitan Club in the District as well as several organizations of Sherlock Holmes enthusiasts.

His first marriage, to Eleanor Owens, ended in divorce. Survivors include his wife of 33 years, the former Julie von Sternberg of Manasota Key, Fla.; five children from his first marriage, Eleanor Earle Mascheroni of New York City, Ralph Earle III of Boston, Duncan Earle of Lusaka, Zambia, Amanda Earle Ciccarelli of Bloomington, Ind., and Caroline Earle Walsh of Barrington, R.I.; a half sister; and nine grandchildren.