Leo C. Nulty, 76, a former FBI special agent and retired congressional investigator who had served as a Fairfax County special magistrate from 1970 to 1978, died of cancer July 18 at Fairfax Hospital. He lived in Vienna.

Mr. Nulty was a native of Kansas and a graduate of the University of Notre Dame. Before joining the FBI in 1938, he worked for the National Youth Administration. He was assistant special agent in charge of the FBI Salt Lake City field office when he transferred to Washington in 1949. Here, he served in its section dealing with sabotage and espionage before retiring in 1952.

For the next 14 years, he worked for several congressional committees, spending most of his time with the permenant investigations subcommittee of the old Senate Government Operations Committee. He retired from congressional work in 1966.

Mr. Nulty was a member of Our Lady of Good Counsel Catholic Church in Vienna and the Society of Former Special Agents of the FBI, and served on the Council on Alcoholism for Fairfax County.

Survivors include his wife, the former Dorothy Boyer of Vienna; a son, A. Timothy, of New Carrollton; three daughters, Patricia Sobota of Kalamazoo, Mich., Mary Anne Nulty of Vienna, and Jeanie Furnari of Lovettsville; a brother, Robert J., of Plymouth, Mich.; two sisters, Kay Conrad of Atchison, Kan., and Betty Hutchison of Hays, Kan., and 10 grandchildren.