ABSTRACT Oceanographic-survey and water-sampling techniques were employed to evaluate the amount and distribution of the fallout received over extended areas adjacent to nuclear detonations of high yields. The project was established as a result of the fallout phenom- ena observed following Shot 1. The operational and technical details had to be hastily contrived so that they could be put into effect within the latter phases of Operation Castle. Specifically, the experimental studies reported herein were conducted in connection with Shots 5 and 6. Immediately following Shot 5, a fleet tug carrying improvised radiographic and oceano- graphic gear cruised the oceanic area downwind of Bikini Atoll, covering 800 miles in four days, taking samples of the water at the surface and to depths of 2,400 fect, and measuring gamma ray intensities above the sea surface and also just below the sea surface. Occasionally the gamma intensity was measured to 80 meters depth also. Two samplesof open-sea plankton were netted and found to be strongly radioactive. Following Shot 6, two tugs cruised downwind of Eniwetok Atoll taking surface water samples and measuring gammaintensity at each level; simultaneously, the area was surveyed by aircraft carrying sensitive gamma detectors. Two survey results recommend the continued use and perfection of the novel techniques. Analysis of data indicates that, for a surface water detonation of a high-yield weapon, an area of approximately 5,000 square miles can be covered by contamination at levels that would be hazardous to human life if the fallout had been deposited on a comparable land area; that is, over this area the total gamma ray dose accumulating during the first 50 hours would be about 250 r at a height of 3 feet above a plane fallout catchment.