ABSTRACT An investigation of the radioactive contamination of the water and marine organisms in and near the Eniwetok Test Site was conducted during May 1958, shortly before and immediately following an underwater nuclear detonation. At the end of three and one-half days the boundaries of the radioactive water mass extended beyond the survey area, 50 miles to the west of Eniwetok Atoll, and to a depth of at least 300 meters. The plankton contained high levels of Np239, Mo99- Tc99mM, Te132-7132) and U237_ Also present in lower amounts were Cel4l-py141 Ru103_Rp 103 Ba!40.7,9140 Zr95-Nb25, and Ce!44pri44, Ru106. pp 106 and Rul05_Rp_105 were detected once in each of two samples, Whole fish samples contained essentially the same radioisotopes as the plankton. Shrimp and squid contained high levels of Np239 and Te132-1132 put no detectable Mo?9-Tc29Mm, nentTy DoT ap 1