4g SUMMARY 1. A survey of the radioactivity in the sea water, plankton and fish was made in September 1956 in an area bounded by 9° N and 15° N and approximately 145° E and 166° E. 2. The general pattern of distribution of activity in the sea shows a sharp decrease in activity east of Bikini and a gradual but irreguler decrease west of Eniwetok. 3. Plankton appeers to be the most sensitive indicator of radioactivity in the sea; the average of ratios of plankton activity to sea water activity was 2,500. 4, The highest value of radioactivity in plankton, 21,000 d/m/g wet, was found about 80 miles north of Fniwetok and the lowest velue, 27 d/m/g, near Guan. 5. The highest value for water activity, 19,009 d/m/l, was found between Eniwetok and Ujelang and the lowest value, 48 afm/l. northeast of Bikini. 6. Sea water filtered through millipore filters had higher levels of activity than the residue on the filter paper. 7. Mteroplankton is probably the principal source of the radioactive particulate matter. 8. The change of relative radioactivity of the water with depth ts variable put appears to fall into regional patterns. at!