Carl J. Haardt, 70, a retired research librarian at the Library of Congress with 33 years of federal service, died Tuesday at Georgetown University Hospital. He had suffered from a heart ailment.

Mr. Haardt began his government career in 1933 with the Department Commerce. He then worked for the Board of Economic Warfare and the Federal Communications Commission before moving to the Library of Congress in 1958.

Following his retirement in 1966 from the library, he worked in the tax assessor's office of Prince George's County until he retired a second time in 1973.

Mr. Haardt was born in Anamosa, Iowa. His family moved to Washington when he was six, and he attended Gonzaga High School here. He graduated from Geogetown University's School of Foreign Service in 1933.

He had lived in District Heights since 1953 and was active in civic affairs there. He also was a member of the Pope Pius X Council of the Knights of Columbus.

Survivors include his wife, Alma, of the home; three sons, C. Joseph Jr., of Springfield, William F., of Houston, Tex., and James E., of the home; three sisters, Betty Kraus, of Arlington, Annabelle Hensley, of District Heights, and Mary Conway, of Forestville; a brother, Henry, also of Forestville, and four grandchildren.