Toxic effects of roxarsone were investigated when carp (Carassias auratus) was exposed to roxarsone solution of 30 and 45 mg/L for 2 and 5 days. Roxarsone showed low toxicity to carp, but the margin of safety was narrow, the LC50 were as follows: 24 h LC50 181.69 mg/L, 48 and 96 h LC50 176.57 mg/L. The activities of LDH in carp brain and carp liver were inhibited significantly (P < 0.05) and showed dose-effect relationship. The contents of MDA in carp brain and carp liver increased significantly in all treated groups (P < 0.05) . Initially the activities of SOD in carp brain, liver and gill were enhanced significantly (P< 0.05) and then inhibited markedly with increasing doses. The activities of GSH-ST in carp brain were inhibited strikingly ( P <0.01) . These results showed roxarsone could inhibit enzyme with sulfhydryl group and the ability of antiox-idation. The oxidative damage to carp were remarkable and showed dose-effect relationship.