The radioactivity of several samples of Coryphaena Hippyrus caught in the southern Pacific in May, 1954, after the atomic explosion at Bikini, was found, in decreasing order, in spleen, kidney, liver, pyloric ceca, heart, gill, intestine, gastric wall, ovary, testis, gastric content, red muscle, skin, vertebrae, and muscle. The red muscle of Neothunnus Macropterus showed 54.8 counts/min./0.20 g. activity on dry basis, the activity was decreased to 27.6 by soaking 25 g. muscle in 25 cc. water, and to 14.1 by soaking in 0.5% Na ethylenediaminetetraacetate solution. The radioactive substances in these fish tissues were found, upon analysis, to belong to the III group, particularly to III-B group. Examination of synchroscope patterns by scintillation counter indicated the presence of /sup 65/Zn among the radioactive substances. /sup 90/Sr was suggested to be present in very small amount.