EPR studies have been performed on some gamma-irradiated marine mussels (Mytilus galloprovincialis) and scallops (Pecten jacobaeus) from the families of Mytilidae and Pectinidae, respectively. Before gamma-irradiation, the EPR lines of Mytilus galloprovincialis indicated the existence of Mn2+ ions, which were not observed in the powders of scallop shells. gamma-irradiation induced defects in powders of Mytilus galloprovincialis shells, were attributed to orthorhombic CO3- axial CO3,3- orthorhombic CO2-, freely rotating CO2-, freely rotating SO2-, axial SO3-, isotropic PO42- and organic free radicals. gamma-irradiation induced defects in powders of Pecten jacobaeus shells were attributed to orthorhombic CO3-, axial CO33-, orthorhombic CO2-, freely rotating CO2-, freely rotating SO2-, and axial SO3-- free radicals. The EPR parameters of the free radicals were compared with literature data on similar defects.