INTRODUCTION Operation Castle was an atmospheric nuclear test series conducted in the Marshall Islands from March to May of 1954. The most notorious test of the series was Bravo, a 15 megaton fl) thermonuclear explosive. The top of the resultant debris cloud reached to nearly 35 km at stabilization time lt! Because of an unexpected shift in mid-tropospherie wind directions following detonation of Bravo, the fallout pattern, instead of heading in the predicted northeast direction, had an easterly alignment. As a result, persons on the atolls of Rongelap and Rongerik were exposed to relatively high levels of fallout from tne niuelear explosion. Prompt action was taken by U. S. Task Foree personnel to evacuate the natives of these islands. Some of the natives on Rongelap, tha closest to the detonation point, suffered temporary nausea and minor skin burns. None exhibited any medium or long term effeets from their exposure. However, after about 10 years, those Rongelap natives, who were young children in 1954 developed non-maligment nodules on their thyroid glands. Since then tne occurrence of similar nodules among the Utirik natives has been reported. The rate of occurrence has been higher than would be expected statistically. The purcose of this report is to calculate deposition and surface air concentration plots, using a three-dimensional particle-in-cell suite of codes to estimate the doses at the isLands from which the natives were evacuated. We will also consider the dose from rainout as part of the debris cloud crossed the atolls. Finally, the calculated time nistory of air concentrations on the downwind islands will be presented for several nuclides.