Abstract: Effects of endosulfan were investigated on activities of Phase Ⅰenzymes(APND,ERND) adn Phase Ⅱ(GST)enzymes in the liver of Ctenopharyngodon idellus and on its DNA were investigated.The experiment was designed to have four treatments,that is CK and three levels of endosulfan concentration(A:0.18,B:0.36 and C:0.71 μg·L^-1).Samples of the fish were taken for analysis separately after they lived in these treatments for 24,72,120 and 168 h.Results show that in Treatments B and C the activity of APND was significantly higher after 24h(P<0.05 or P<0.01),but inhibited significantly after 72h(P<0.01)as compared to CK.It was inhibited to a significant extent after 120 h in all the treatments (P<0.05 or P<0.01).On the whole the activity of ERND was induced by the exposure,whereas the activity of GST was induced at the initial stage and then turned to be inhibited.In Treatments B and C the activity of GST peaked after 72 h of exposure and afterwards declined gradually with the duration of exposure,till 168 h when it turned to be inhibited,while in Treatment A,it peaked after 120 h and declined afterwards,too,till 168 h when it turned off to be equal to that in CK.The damage the exposure of the fish to endosulfan brought to DNA of its liver cells was significant.TL and %TDNA as in dicators of DNA damage increased with the increasing dosage of endosulfan in treatment or the extending duration of exposure,showing a significant relationship.In conclusion,endosulfan affects activities of Phase Ⅰand Phase Ⅱenzymes in the liver of Ctenopharyngodon idellus and causes genetic damage to the DNA of liver cells of the fish.