-21- On may 14, 1958, a second fallout was detected in the area in which the ship was operating. This contamination may have resulted from the Butternut, Fir, or Koa devices which were detonated one to to two days earlier. General contamination of the ship at a level of 5 to 10 mr/hr raised the background in the counting room to a very high level (445, 000 c/m total gamma, 0.02-2 MEV). This level decreased to 60,000 c/m just prior to the Wahoo detonation on May 16, 1958. The first post- Wahoo plankton sample, P-13, was taken at the end of the initial radiation survey (1609 hours) at a depth of 9 to 15 meters in an area with a gamma radiation level of 10 mr/hr, four miles west of target zero. The amount of radioactivity was high (32, 000, 000 d/m/g dry) but not as high as that found in sample P-14 taken that evening (1907 hours) at a position less than three miles to the southeast. These differences in amounts of radioactivity may reflect the variation in the radioactivity of samples taken in the target area or mayindicate that maximum uptake of radioactivity by the plankton did not occur until this time. The location of the plankton sampling stations and the radio- activity of the samples collected after Wahoo are shown in Figures 1, 5, and 6. The results of ion-exchange separations of the radionuclides of some of the plankton samples and analysis of the fractions by gamma spectrometry are given in Table 3. The percentages of total activity