To examine the radionuclide concentrations in the environment, seaweeds were collected once a year since 1976 at two different places. A marked change was observed in the number of radionuclides in seaweeds. The increase of radionuclides is attributed to contamination by radioactive fallout from Chinese nuclear explosion tests. The radionuclide concentrations were strongly dependent on the species of seaweeds. A gradual decrease of the137Cs/40K activity ratio was observed in spite of the input of additional137Cs fallout from Chinese nuclear tests. This implies that the amounts of137Cs introduced into seawater by these detonations were small in comparison with those already contained in seawater. The cesium content in seawater was estimated by using specific activities of137Cs in seaweeds and137Cs concentration in seawater. The results show good agreement with the reported values by direct analysis of seawater.