ABSTRACT The Marine Survey Unit had as its major objectives (1) the measurement of the residual radiation of Eniwetok Atoll as found in the living organisms as a result of previous weapons tests in this area, and (2) a resurvey of the area, following the Mike shot, to determine the change in amounts, kinds, and distribution of radioactive materials. The field data were collected by a party of seven specialists who spent the time from October 20 to November 11, 1952, collecting plankton, algae, rats, birds, fishes, plants and invertebrate organisms. The material collected was frozen for storage and shipment back to the Applied Fisheries Laboratory where it was identified, dissected, weighed, ashed, and the amount of radiation measured as disintegrations per minute per gram of wet sample. The pre-test survey showed measurable amounts of residual radiation on and in the living organisms collected from the stations along the eastern and northern portion of the atoll. Following the Mike shot the radiation level increased manyfold, especially along the northern and western portions of the atoll. It was found that some of the organisms collected and studied had been exposed to sufficient external and/or internal radiation to eventually damage or destroy them. Subsequent studies should determine the biological half life of the materials contaminating the area, their shift in position with the currents, and the results of the contamination from radioactive materials upon the living forms of the atoll.