Abstract: Adult freshwater fish, Oreochromis mossambicus was exposed to nonylphenol at sublethal concentration (0.15 mg/ L) for 24 h, 96 h and 7 days. Nonylphenol significantly decreased the weights of liver and also hepato-somatic index at the end of 7 days, however no such changes were observed at 24 and 96 h of treatment. Decreased weight may be due to necrosis or atrophy of hepatocytes, which is evidenced by the histopathological observations. The activities of antioxidant enzymes decreased after 24 h and 96 h whereas at 7 days of exposure the activities of antioxidant enzymes begin to increase and the levels of hydrogen peroxide and lipid peroxidation decreased to normal level like that of control groups. In general, repeated doses of nonylphenol for 7 days, may induce a defensive response in Oreochromis meanwhile, acute exposure caused inhibition of antioxidant activities. The present study also showed a significant decrease in the marker enzyme, alkaline phosphatase after 96 h indicating the decreased state of inter and intracellular membrane transport and the activity of enzyme increased to normal level at 7 days. In histopathological observation, the control liver exhibited a normal architecture with hepatocytes presenting a homogenous cytoplasm and a large central or subcentral spherical nucleus. In the treated group an increase in cytoplasmic vacuolization, necrosis and decrease in the number of hepatocyte nucleus was observed. This could be possibly owing to the toxicity of nonylphenol and also due to the generation of reactive oxygen species in fish hepatocytes.