ABSTRACT A survey of the radioactivity in the sea in the region of the North Equetorial Current from the Marshall Islands to the Marianas Islands was made in September 1956. The expedition W&S sponsored by the United States Atlomic Energy Commission, Division of Biology and Medicine, and carried out by the Applied Fisheries Laboretory, University of Washington, with the support and cooperation of the United States Navy. Plankton samples were taken by oblique tows from 200 meters and water samples were taxen from the surface, 150 meters at 74 stations. 25, 50, 100, and The general pattern of distribution of radioactivity shows a sharp decrease east of Bikini and a gradual but irregular decrease west of Eniwetok. A slight de- gree of contamination is indicated as far to the west as Guam, the western extremity of the survey. Non-fission products account for a large proportion of the radioactivity in plankton and fish samples.