Phytoplankton (platymonas sp.)，rotifer (Erachionus plicatilis Miiller)，clam (Area granosa Linnus)，shrimp (Penaeus penicillatus Alcock) and fish (Tilapia mossambica Poster) were respectively. cultured in sea water with sup>50Co (5 x 10^-9Ci/l) and sup>137Cs (5 x 10^-8 Ci/l) The accumulativ a ability of radioactive substance in different organism and the distribution of these radioactive substance in accumulated organism were studied, Ge(Li) gamma-ray detector and S-80 type multiJanalyser were used for measuring radioactivity of samples. It was found that in如ytoplanktan，rotifer, clam and fish the accumulativ e ability of sup>50Co is greater than that of sup>137Cs. The critical organism for accumulation of sup>50Co is phytoplankton and clam, and that of sup>137Cs is pb.ytoplaxWtvn and shrimp. In fish the accumulativa organs of sup>50Co and sup>137Cs are stomach, intestine and liver.In shrimps accumulative organs of sup>50Co are head and chest, and sup>137Cs distributes in the whole body of the shrimp. The different results on degree of accumulation of sup>50Co and sup>137Cs in every organ of clam，which are obtained by two tracer mathods, i.e. with single nuclide, sup>50Co or sup>137Cs, and with both the nuclides, may be due to the variant experimental period in our studies.