Tremendous use of organophosphate insecticides especially chlorpyrifos have led to elevation of the residual levels of these insecticides in aquatic system via contaminated runoff and atmospheric deposition. Our study was done to detect the effect of different chronic concentrations of chlorpyrifos (¹/₁₀ and ¹/₂₀ of 96 h LC₅₀) on fingerlings Nile tilapia, Oreochromis niloticus, after 8 weeks of exposure through determine the oxidative potential indices, immunological parameters and hepatic and renal markers in serum. DNA damage was determined by single-cell gel electrophoresis (Comet assay). The marked change in clinical signs and postmortem findings were recorded in chlorpyrifos exposed groups. The results showed significant increase in malondialdehyde (MDA) levels, decline in antioxidant parameters which represented by decreasing in Glutathione (GSH) activity, inhibition in immunological parameters which represented by decrease in lysozyme and nitric oxide levels in the groups that were exposed to ¹/₁₀ and ¹/₂₀ of 96 h LC₅₀ of chlorpyrifos. The significant elevation in hepatic and renal markers which represented by increasing in Alanine transaminase (ALT) and creatinine activities and a marked elevation DNA damage were recorded in the groups that were exposed to ¹/₁₀ and ¹/₂₀ of 96 h LC₅₀ of chlorpyrifos. We conclude from our study that chronic exposure of juvenile O. niloticus to chlorpyrifos has the potential to cause harmful effect.