The measurement of radioactivity of the surface water (initial radiation survey) was conducted with the assistance of personnel from the Laboratory of Radiation Biology (University of Washington) and from the Radiation Safety Section of the Los Alamos Scientific Laboratory under the direction of Maj. Gordon Jacks. The radiobiological survey extended from 10° 56'Nto 11° 25'N and from 161° 16' Eto Atoll (Fig. 1). 162 10! E, the area southwest of Eniwetok The survey included thirty-five plankton stations and four water stations. during the period. Eight mid-water trawl collections also were made Details of these coNections describing location, depth of sample, temperature, and depth of thermocline are given in Table 1 and Appendix Tables A, B, and C, and the locations of the stations are shown in Figures 1, 2, and 3. Details of the work aboard ship and the methods of collection were similar to those’ described by (2) (14) Donaldson et al. and Seymour et al. MATERIALS AND METHODS Piankton Plankton samples were collected with a 1/2-meter plankton net of No. 6 mesh while the ship was drifting at a rate of approximately 0.5 to 1.8 knots per hour. The net, hauled by a 3/16-inch wire cable ona