The activity of Ethoxyresorufin-o-dealkylase (EROD) in the liver of Oreochromis niloticus and Clarias gariepinus was evaluated as a response to experimental and natural contamination of water with Benzo-a-pyrene and/or cadmium. The activity was measured fluorimetrically in the hepatic S9 fraction while the content of the enzyme was measured by ELISA. The response appeared as early as six hours post exposure. This study also reveled that Oreochromis niloticus exhibits higher values of EROD activity than that of Clarias gariepinus. CYP450 1A1 content showed lower responsiveness when compared to EROD activity measurements. The present study also estimated the inhibitory effect of cadmium on CYP450 1A1 induction. The current results demonstrate that EROD activity reflects contamination of water with benzo-a-pyrene as a polycyclic aromatic hydrocarbon compound. Consequently it is a useful biomarker for monitoring this type of pollution.