INTRODUCTION Operation Castle was an atmosoheric nuclear test series conducted ia tnz Marshall Islands from Mareh to May of 1954. The most notorious test of the series was Bravo, a 15 megaton (1) thermonuclear explosive. The top of the resultant [1] debris cloud reached to nearly 35 km at stabilization time. Because of an unexpected shift in mid-tropospherice 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 Rongertk were exposed to relatively high levels of fallout from tne nuclear explosion. Prompt action was taken by U. S. Task Force personnel to - evacuate the natives of these islands. Some of the natives on Rongelap, the closest to the detonation point, suffered temporary nausea and minor skin burns. None axnidited any medium or long term effects 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 purpose 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 cainout as part of the deoris cloud crossed the atolls. Finally, the calculated time ststory of air concentrations on the downwind islands will be presented for several nuclides.