Phthalates, commonly used as plasticizers in PVC products, are frequently released into the aquatic ecosystem causing adverse toxic impacts on the inhabitants. The hypothesis of the present study was to investigate the toxic effects of one of the phthalates, diisononyl phthalate (DINP) on the antioxidant status in gill, liver and mus-cle tissues of the fish, Oreochromis mossambicus. DINP at 300ppm concentration was exposed to fish for short-term (24, 48, 72 and 96 h) and long-term (7, 14, 30 and 60 days) durations maintaining vehicle control (propylene glycol as solvent) and negative control (without solvent) groups.In gill tissue, the activities of antioxidant enzymes such as superoxide dismutase, catalase, glutathione reductase and glutathione peroxidase showed significant (P<0.05) in-crease initially during short-term exposure followed by significant (P<0.05) reduction in all treatment groups of long-term exposure. The level of lipid peroxidation and the activities of acid and alkaline phosphatase increased significantly (P<0.05) after DINP exposure in all treatment groups. Liver tissue showed significant (P<0.05) reduc-tion in the activities of all antioxidant enzymes except glutathione reductase in all treatmentgroups with concomi-tant increase in the level of lipid peroxidation and the activities of alanine and aspartate aminotransferase. Similar-ly, reduction in the activities of antioxidant enzymes and induction of lipid peroxidation along with significant (P<0.05) increase in the marker enzymes were observed in the muscle tissues. The study revealed that DINP induced toxicity by the alteration of antioxidant status in gill, liver and muscle tissue. The patterns of variations observed in different tissues are duration dependent and also indicate that all tissues are equally susceptible to oxidative stress by the exposure to the toxicant.