Abstract: Effects of exposure to different concentrations of benzo(a)pyrene(BaP)(0.1,1,10 and 50 μg•L^-1)on cytochrome P450 1A1 (CYP 1A1) and glutathione S-transferase (GST)activities in liverof tilapia were studied.Results show that the exposure to BaP, 0.1,1 or 10 μg•L^-1 in concentration did not have significant effect (P > 0.05) on CYP1A1 activity, but when the concentration rose up to 50 μg•L^-1, CYP1A1 activity was significantly induced after 6 or 168 h of exposure (P < 0.05). Likewise, the exposure to BaP, 0.1 or 1 μg•L^-1 in concentration did not have significant effect on GST activity in liver(P > 0.05), and when the concentration rose up to 10 or 50 μg•L^-1, it did after 336 h of exposure (P < 0.05). The study on relationship between CYP1A1 and GST in activity reveals a similar trend in variation, especially in groups with BaP concentration being 1 or 50 μg•L^-1, suggesting that there exist certain correspondence between CYP1A1 and GST activities.