[en] The radioactivity concentrations, concentration factor and its accumulation in marine organisms were described. The radioactivity concentration of ¹³⁷Cs in seaweed in coastal areas of Japan changed from 5Bq/kg-fresh to 0.1Bq/kg-fresh in 1964 and 1990's, respectively, without the high value in 1986 by Chernobyl accident. The concentrations of ¹³⁷Cs in fish in the northern hemisphere were larger than in the southern hemisphere and the values are decreasing year by year. Recommended values of the concentration factors of 60 elements on macro-algae and surface-water fish (IAEA, 1985) are shown. The mechanism of uptake, excretion and accumulation of radionuclides by marine organisms are explained by some examples such as Holothurodea, Palaemonidae, Octobrachiata and Seriola. (S.Y.)