Chlorpyrifos (ChF) is widely used in agricultural fields and indoor for controlling pests. It sinks in the aquatic sediments and pose toxicity for aquatic organisms including fish. Therefore, the current study was designed to investigate the oxidative stress enzymes, and histological alterations in the liver, spleen, and heart of tilapia (Oreochromis niloticus) due to ChF exposure. Median lethal concentration (LC50 24 H) was calculated as 52.78 ?g/l by exposing fish with different acute concentrations (0, 1, 15, 35, 40, 45, 50, 55, 100 and 150 ?g/l) of ChF. For assessment of sublethal toxicity of ChF, the fish were divided into four groups, one group served as control (C) while three groups (ChF1, ChF2, ChF3,) were exposed to three sublethal doses of ChF. ChF1 group was treated with 3.51 ?g/l (1/15th of LC50) of ChF whereas, ChF2 and ChF3 groups were treated with 5.27 ?g/l (1/10th of LC50) and 10.55 ?g/l (1/5th of LC50) of ChF respectively for 14 days and oxidative stress enzymes, biochemical parameters, behavioral changes and histological alterations were evaluated. It was found that level of Glutathione-S-Transferase (GST), Catalase (CAT), and Superoxide dismutase (SOD) increased significantly (p=0.05) in the gills and liver of tilapia treated with sublethal concentrations of ChF. Study of morphological features exposed that fish in the control groupshowed significantly increase in weight and length (p=0.05) as compared to ChF treated fish. The behavioral changes as reduced feeding rate, eratic swimming, increased surface gulping, aggregation in the corner of tank, hemorrhage, and darkening of skin coloration was observed in all ChF treated groups. The significant elevation in level of aspartate aminotransferase (AST) and alanine aminotransferase (ALT) was observed in all groups treated with different sublethal concentrations of ChF. Histological study of liver tissues shown increased number of kupffer cells, hydropic degeneration, dilated sinusoids, congestion, necrosis, and hemorrhage in liver of fish from all treatment groups. In the spleen of fish exposed to ChF increased number of melanomacrophage centres (MMCs), necrosis, congestion, and hemorrhage was detected. Disorganized muscle fibers, cardiac muscle fiber degeneration, coagulative necrosis, congestion in blood vessel and necrosis was perceived in the heart of ChF treated fish. This study concluded that sublethal concentrations of ChF can induce oxidative stress and histological alterations in the tissues of tilapia.