Morphological alterations and nuclear abnormalities in fish erythrocytes have been used in many studies as bio-indicators of environmental mutagens including ionizing radiation (IR) irradiation. In this study, adult Japanese medaka (Oryzias latipes) were irradiated with gamma-rays at a low dose rate (9.92 µGy/min) for 7 days, giving a total dose of 100 mGy, and morphological alterations, nuclear abnormalities and apoptotic cell death induced in peripheral erythrocytes were investigated 8 hours and 7 days after the end of the irradiation. A variety of the abnormality, such as tear-drop cell, crenated cell, acanthocyte, sickled cell, micronucleated cell, eccentric nucleus, notched nucleus, and schistocyte, were induced both in the peripheral erythrocytes of the wild-type fish and a less number of the abnormalities and apoptotic cell death were induced in the p53-deficient fish. These results indicate that low dose-rate chronic irradiation of gamma-rays can induce cytotoxic and genotoxic effects in peripheral erythrocytes of medaka and p53-deficient medaka are tolerant to the gamma-ray irradiation than the wild-type on the surface