Previous studies have examined the effects of gamma-radiation on Japanese fish, in particular medaka(Oryzias latipes). In the present work, alterations in erythrocytes were recorded as haematological bio-indicators of exposure to gamma-radiation. After exposure of medaka fish to two different doses ofradiation (2 Gy and 10 Gy), many malformations in red blood cells were observed in the irradiatedfish compared with control fish. These malformations included acanthocytes, crenated cells, amoeboidcells, and sickle cells. More malformations were seen at the higher radiation dose. No micronuclei wereseen in any group, but nuclear abnormalities were observed. We conclude that gamma-radiation causesmorphological malformations of erythrocytes and is harmful to medaka fish.