The aim of this study was to investigate the propensity of chlorpyrifos (CPF) to induce formation of protein carbonyl (PC) in the liver and genotoxic effects in peripheral erythrocytes of Oreochromis niloticus by using micronucleus (MN) and nuclear abnormalities (NAs) tests. Also, possible attenuation by vitamin C (Vit C) on these parameters was investigated. Fish were exposed to 12 and 24 mu g/L chlorpyrifos for 96 hours. The liver of O. niloticus, exposed to 24 mu g/L CPF, showed higher protein carbonyl levels than control. Administration of vitamin C was effective in reducing PC level in CPF+Vit C group. The MN and NAs frequencies in peripheral erythrocytes of fish were observed to be increased depending on CPF dose applied. Administration of Vit C ameliorated the increase in formations of MN and NAs. These data indicated the protective role of ascorbic acid against chlorpyrifos-induced genotoxicity and suggested a significant role of its antioxidant property to these beneficial effects.