Changes of aryl hydrocarbon hydroxylase(AHH) activities in the liver of Boleophthalmus pectinirostris with benzo ( a ) pyrene ( BaP) exposure were detected in experimental condition. The results showed that the activities of AHH were significantly induced by BaP ( P < 0. 01) . Factors including BaP concentration and exposure time had important effect on AHH activities. The activities of AHH decreased to the control level after BaP was removed. It turned out that the physiological modulatary mechanism was still exist in the liver of B. pectinirostris and the activities of AHH could reflect the level of BaP in the environment. All of the results showed that AHH was suitable to be the bioindicator of BaP exposure.