Serious problems of pollution and health hazards accompanied the wide range of production and application of pesticides during the last few years had occurred. Ninety Oreochromis niloticus were exposed to 1/₁₀ and 1/₂₀ 96 hrs LC₅₀ to assess its chronic injurious effect on growth performance, biochemical analysis and histopathological alteration. The results revealed that a significant decrease in the final body weight, weight gain and body gain percentage and condition factor. The usual inhibition of Acetylcholine esterase (AChE) was detected that result in the detected behavioral changes. There was significant increase in superoxide dismutase enzyme activity (SOD), malondialdehyde (MDA), alanine aminotransferase (ALT), creatinine and urea levels with significant reduction in serum glutathione (GSH) level  and total serum Immunoglobulon M (IgM). The detected histopathological alterations in liver, kidney and gills were related to the concentration and duration of exposure to Fenitrothion.