Harold Jefferson Coolidge, 81, an internationally known zoologist and conservationist and the former executive secretary of the Pacific Science Board of the National Academy of Sciences, died Feb. 15 at his home in Beverly, Mass. of pneumonia and complications after a fall.

Mr. Coolidge, a former curator of mammals at Harvard's Museum of Comparative Zoology, coordinated scientific research in the Pacific for the National Academy of Sciences from just after World War II until he retired in 1970. He was also a founder of the U.S. World Wildlife Fund, and he was a trustee and founder of the Darwin Foundation, which conducts scientific activities on the Galapagos Islands in the Pacific Ocean off the coast of Ecuador.

He was a founder and president of the International Union for the Conservation of Nature and a secretary of the American National Parks Association.

Mr. Coolidge was born in Boston and graduated from Harvard. He was on the staff of the Museum of Comparative Zoology from 1929 to 1946, even after moving to Washington in 1940 to join the Office of Strategic Services. He remained with OSS throughout World War II, then joined the National Academy of Sciences.

He was a member of the Cosmos Club, and in 1980 became the first American recipient of the J. Paul Getty medal for wildlife conservation. He also received the Gold Medal from the New York Zoological Society and the Browning Medal from the Smithsonian Institution. He lived in Washington until 1979 when he returned to Beverly.

His marriage to Helen Isaacs Coolidge ended in divorce. Mr. Coolidge is survived by his wife, Martha Henderson Coolidge of Beverly; three children of his first marriage, Nicholas Jefferson Coolidge and Isabella Gardiner Coolidge, both of Washington, and Thomas Richards Coolidge of New York City; a sister, Mrs. Thomas A. Stone of New York City, and six grandchildren.