ABSTRACT The objective of this project was to document the distrilution and intensity of fallout from all shots at Operation CASTLIL., Data were obtained for Shots 1, 2, 3, 4, and 6 by use of land stations, anchored lagoon stations, and free-floating sea 3ta%ions. A complete analysis of the Shot 1 fallout to 300 nautical miles downwind including the development of an experimental model based on fallout particle trejectories is presented as well as data on Shot 2 fallout to * nautical miles downwind and the close-in fallout from Shots 3, 4, and 6, Gamma fields from fallout decayed at rates differing from the t71+2 approximation commonly applied to fission weapons. Fallout from the surface land detonations was in the form of irreg- ular solid particulates. The geometric mean particle diameter decreased with the distance from the shot points; for Shot 1 the geometric mean varie from 112 } at Bikini Atoll to 45 pat Utirik Atoll. The average density of the solid particles from Shot 1 was 2,36 g/cu cm. . Little data were obtained on the nature of the fallout from over-water detona~ tions. There was some indirect evidence that the fallout 50 nautical miles downwind from Shot 2 arrived as a fine mist or aerosol. The rate of arrival of fallout at distances close to surface zero was character~ ized by a rapid rise to a peak; the maximum level of radiation occurred within the first half of the period of fallout. A continuous 100 hr unshielded exposure after the detonation of a 15-Mf device on land, will result in a minimum free field total dose of 100 r over an area as large as 25,000 sq mi. . There is developed an experimental model that provides a means of reconstructing fallout patterns from limited gamma field data and particle trajectories as determined by comprehensive analyses of the meteorological situation.