Chlorpyrifos (CFS) is one of the most common insecticides found in freshwater ecosystems and has been detected in agricultural and fishery products worldwide. Acute toxicity assessment was carried out by24hour LC50 of chlorpyrifos to freshwater fish Oreochromis niloticus. The value for 24hour LC50 was determined as 37.93 ? 20.02?g/L. The experiment was carried to measure the effects of different concentrations of chlorpyrifos (CFS) on O. niloticus through the evaluation of biochemical and hematological parameters in addition to histological examination. One hundred and twenty O. niloticus were randomly distributed into four groups in triplicates containing ten fish per aquarium. The first group served as a GC (control group), which is the non-treated group. The second group G1 was exposed to 1/11 of 24hour LC50. The third groupG2 was exposed to 1/7th of 24hour LC50. While the fourth group G3 was exposed to1/3 rd of 24hour LC50.The exposure was for 14 days in all treated groups. Fish exposed to CFS showed histological alterations as compared to control groups. Alterations were more prominent in high dose group G3. Congestion, necrosis and hemorrhage in gills, degeneration in muscles and intestine were noticed. In hematological assessment, significant increase in WBCs, LYMP,MCH and MCV with the increase in dose was noticed while decrease in RBCs, HGB, HCT and MCHC was observed with increase in dose as compared to control. Significant increase in AST and ALT was observed.This study provides significant perceptions about the acute toxic effects of CFS on fish and show potential to be used as biomarkers in further toxicological evaluation studies