Harry Stephen Ladd (1899-1982) received his Ph.D. in geology from the University of Iowa in 1925. The initial part of his professional career was spent in faculty positions at his alma mater and the University of Virginia, as a geologist with the Gulf Oil Company in Venezuela, and studying fossils under Edward Oscar Ulrich at the United States National Museum. Ladd began his career with the United States Department of Interior in 1936 when he was appointed Geologist with the National Park Service. In 1940, he joined the United States Geological Survey (USGS) as a Geologist. He remained with the USGS until his retirement in 1969. Ladd was a Research Associate in the Department of Paleobiology, National Museum of Natural History from 1969 until his death. Ladd was an authority on the geology of the islands of the Pacific Ocean. He conducted extensive field work in the Pacific Basin and supervised deep sea drilling on Eniwetok, Midway and other atolls. He was also a major contributor to Project Mohole, a plan for drilling deep into the earth's inner crust on the Pacific Ocean floor near Hawaii. During 1946-1947, Ladd was in charge of geological work on Crossroads Operation which conducted the scientific survey and resurvey of Bikini Atoll, the site of atomic bomb tests. Ladd was active professionally and was a vice president of the Geological Society of America, 1955, and the American Association for the Advancement of Science, 1965, and President of the Paleontological Society, 1954.