BS>Genetic analyses were made of irradiated and control isolated populations of D. ananassae from the Pacific Proving Ground area, others of the Marshall Islands, and Ponape. These tests shows that the Bikini and Rongelap populations were severely damaged genetically by the direct radiations and fallout from the thermonuclear device exploded on March 1, 1954. The Rongerik atoll populations had recovered from fallout by August 1955 -- a period roughly equivalent to 26 generations. The populations collected at Rongelap and Bikini had not recovered then, but the Rongelap populatlon had recovered by 1956, in 40 or more generations. Because of the heavier irradiation on Bikini--including effects of the weapons tests in 1956--this population was still slightly below other Pacific Island populations even in 1957. The general relations and comparisons between these isolated island populations and other Drosophila- populations are discussed. (auth)