Fish responses have been used as biomarker of aquatic pollution. The objective of this study was to determine some biochemical responses in Oreochromis niloticus after exposure to sublethal concentrations of three different pollutants . In the present study ,malathion (1.75mg/L), (organophosphorus insecticide ) ,lannate ( 0.65 mg/L) (carbamate insecticide) ) and phenol (9 mg/L) (phenolic compound) were selected as model compounds of three different pollutants. Studies were carried out to determine the effect of these compounds in acetylcholinesterase enzyme( AchE) as a biomarker of effect and glutathione S – transferase ( GST) enzyme as a biomarker of defense in brain , serum and liver of O . niloticus after three weeks of exposure. The results indicated that : 1-the activity of acetylcholinesterase enzyme decreased significantly in brain ,serum and liver of O. niloticus exposed to malathion and lannate after 1, 2 and 3 weeks when compared with the control group, phenol treated group has low effect. 2- the activity of glutathione S- transferase decreased significantly in brain and serum of O. niloticus exposed to malathion and lannate , but in phenol treated group the activity of glutathione S – transferase increased significantly in brain and serum. 3-the activity of glutathione S- transe ferase increased significantly in liver of O. niloticus exposed to phenol more than malathion and lannate.