CHAPTER 1 INTRODUCTION As a result of a nuclear detonation in Operation CASTLE, several atolls in the Marshall Islands were accidentally contaminated by radioactive fall-out to such an extent that evacuation of the inhabitants was necessary.} Plants, animals, water, and soil were collected from the islands one month after the fall-out occurred. The animals not sacrificed at the site were sent to this laboratory and radiochemical analysis was made of their tissues to provide information on the internal radiation hazard. A report? of this study has been published. As a follow-up to the original study a resurvey of the contaminated Marshall Islands was undertaken one year after the fall-out. Radiochemical analysis of food plants, fish, water, soil, coral, algae, and birds was made to determine the nature and extent of the internal and external radiation hazard created by the residual contamination on the islands. A gamma dose-rate survey was conducted to determine the external radiation hazard extant. Such data were necessary to determine the possibility of re-occupying the islands. The present report presents the data obtained from the resurvey of the contaminated islands. 1.l OBJECTIVES This work was named the Atoll Resurvey Project, entitled 'Followup Determination of the Extent and Distribution of Fali-out Contamination on Rongelap and Other Atolls in the Marshail Group." Its specific objectives were: a. To provide data upon which a decision can be based as to when the evacuated islands may be safely re-occupied by their former inhabitants. b. To provide information about distribution of the residual contamination on a land area which had been heavily contaminated by fall-out.