Accumulation and depuration kinetics in the samples of mussel (Mytilus galloprovincialis L. 1758) and sea snail (Rapana venosa (Valenciennes, 1846)) exposed to Cs-137 radioisotope via seawater exposure were investigated under laboratory conditions. Accumulation and depuration kinetics were monitored for 49 and 59 days, respectively. Concentration factor and biological half-life values were calculated from the accumulation and depuration kinetics of mussel and sea snail samples. Concentration factors at equilibrium were 16.25 +/- 4.41 and 19.50 +/- 1.17 for large and small mussels, respectively, while 16.34 +/- 0.70 and 14.31 +/- 0.63 for large and small snails, respectively. The biological half-lives at slow components were 38.50 and 77.00 days for large and small mussels, respectively, while 49.50 and 57.75 days for large and small snails, respectively. Compared to the literature, the slightly higher concentration factor and biological half-life values obtained in this study for both mussel and snail samples may help to identify these species as bio-indicators of the Cs-137 radioisotope in the marine environment. In addition, it was statistically determined that body size is an important parameter for both living organisms in bioaccumulation experiments unlike the depuration experiments. In the light of all these results, it has been determined that mussel and sea snail organisms are extremely valuable indicators in terms of reflecting radioactive pollution in the sea in monitoring the Cs-137 radioisotope for possible inputs of any radioactive pollution in the marine environment.