portion of the total radioactivity. Gamma spectra obtained from the sa@me samples, however, indicated that non-fissionproduct radioisotopes contributed more of the total radioactivity than was observed in the chemical separations, which were designed primarily for determination of fission products. Consequently a detailed study was made to de- termine the radioisotopic content of several plankton In addition, samples collected during the September survey. gamma spectra were determined for these and most other plankton samples collected in the latter survey. The re- sults are reported in this paper. Materials and Methods The equipment and methods used for collecting the samples at sea have been described in previous reports (6, 21).— The initial counting for beta activity was done aboard ship soon after collection of the plankton samples, and the remainder of the plankton from each station was preserved in 10 per cent neutralized formalin in sea water and returned to the Seattle Laboratory. DOE ARCHIVES The concentration of plankton in the tropical seas and the levels of radioactivity in the plankton are both limited. thus plankton samples from adjacent stations were combined to provide sufficient material for analysis. Figure 1 shows the track of the September survey and the groups of stations