This study was conducted to investigate the impact of nutritional environmental factors on the efficacy of microsomal biotransforming enzymes to organic xenophobic in the freshwater fish O. nilolicu exposed to the pesticide quintozene Two groups of fish, one was maintained on a normal diet (A) and the other on a diet supplemented with fish liver oil (B) for 4 weeks. Then the fishes were treated with quintozene by interperitonial injection of $50 |il (30 mg/kg of body weight) and the responses of liver microsomal biotransforming enzymes were assessed after 48 hours.Treatment with quintozene of fishes maintained on diet B. produced an increase in microsomal contents of cytochrome P-450 and cytochrome b$ reaching 1.44 and 2.88 fold respectively compared with group A, whereas, activities of 7-ethoxyresorufin -O- deethylase (EROD) and 7- ethoxycouma'rin- deethiase (ECOD) reached 2.8 to 5.11 fold respectively.Similarly, the phase II enzymes, Uridine diphosphoglucuronyl - transferase (UDPGT) activity was less induced in fishes fed diet A than diet B; whereas an increase of 1.64 and 4.7 fold respectively were noticed. Cytosolic glutathione- S- transferase (GST) was not affected by either treatments.In addition, a fish liver oil rich dietary supply of polyunsaturated fatty acids (PUFAs) may prevent these changes, even in a long- term subchronic exposure.The results indicated that fatty acid composition of diet of Oreochromis nilodcus can affect significantly the biotransforming enzymes in response to xenobiotic pollutants.