Accumulation of radiosilver108mAg and110mAg in oysters (Crassostrea gigas) and their behavior in marine environments has been studied in the northeast Pacific coast in Japan. Enrichment of radiosilver in oysters depends on topographical conditions; significant bioaccumulation occurred in open bays, while it was hardly observed in bays with narrow shaped entrances. From these observations difference of the behavior of radiosilver between open and nearly closed bays is suggested.110mAg in oysters decayed with an effective halflife of about 150 days for both the Chinese nuclear weapon test and the Chernobyl accident. In contrast to radiosilver, the fission product nuclide137Cs was almost independent of topographical conditions, and its concentration was constant.110mAg bioaccumulation in oysters after the Chernobyl accident in 1986 was found in both open and nearly closed bays, the latter showing much lower concentration of radiosilver than the former. Specific activity of108mAg in oysters was determined in bays open to the Pacific Oceans.