INTRODUCTION Following the 1954 and 1956 tests of nuclear devices at the Eniwetok Test Site, investigators from both Japan and the United States reported the occurrence of several neutron- induced radioisotopes in marine organisms from that area (Kawabata, 1954; Rinehart et al, 1955; Yamada et al, 1955; Weiss and Shipman, 1956; Amano et al, 1956; Seymour et al, 1957). In later analyses of plankton and fish samples for radioactive isotopes the non-fission products zn©5, C060 , Fe>>, Feo9, Co’, C058, and Mn 34 were found to account for 50 per cent to 99 per cent of the total radioactivity of the individual samples. However, trace amounts in soils, these isotopes were present in only land plants, from the same general area. and terrestrial animals In addition to the neutron- induced radioisotopes the following fission products were present in the plankton and fish samples: Nb29, RuL06_ Rh106, Zr? and ce l44_ ppl 44 The investigations on the uptake and retention of radio- isotopes by plankton and fish at the Eniwetok and Bikini test site were continued during the 1958 atomic test series. period from August 8 to 14, In the one of three oceanic radiological surveys was made by members of the Laboratory of Radiation Biology to measure the amounts of radioactivity in sea water