1. During July 1957, chemical separations by lton-exchange 
resin columns were made on five grouped plankton samples taken 
in and west of the Eniwetok Proving Ground in September 1956. 
Also, radiochemical separations for Sr90-y90 were made. The 
radioactive isotopes in the separated fractions were identi- 
fied by decay rates, maximum beta energies and gamma energies. 
2. The distribution pattern of CoD? and Zn©5 in plankton 
samples not subjected to ion-exchange separation was deter- 
mined by analysis of gamma spectrum curves. 
3. The predominant fission product radioisotopes found were 
Zr95-Nb?>, Celt4_ ppl 44 and in one sample, Rul06_Ry106 , 
Fission products accounted for 22 to 44 per cent of the total 
radioactivity at the collection date. Sr90-y¥90 was not found 
in the ion-exchange separations nor in fuming nitric acid pre- 
cipitations made on duplicate aliquots. 
4, The non-fission radioisotopes Fe>>, Cool, CoD8 , and zn©5 
contributed 55 to 77 per cent of the total radioactivity, and 
Co©° ang Mn54 accounted for a total of 0.8 to 1.6 per cent. 
5. Co57 and Co60 were present at Bikini Atoll in a 1:1 ratio 
and at Eniwetok in a 5.3:1 ratio. These ratios were re- 
flected in plankton samples collected in the vicinity of the 
two atolls. The ratio of Co57/Co®° tn an area 300 miles wide 
north to south and 200 to 500 miles west of Eniwetok was 
