portion of the total radioactivity. Gamma spectra obtained 
from the same samples, however, indicated that non-fission- 
product radioisotopes contributed more of the total radio- 
activity 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 
samples collected during the September survey. In addition, 
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 
(an ROS 
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. 
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 
