The pyrethroid class of insecticides, including deltamethrin, are being used as substitutes for organochlorines and organophosphates in pest-control programs because of their low environmental persistence and toxicity. Ecotoxicological consequences of deltamethrin, particularly its effects on antioxidants in fish and other aquatic organisms, are not well understood. We investigated the effect of deltamethrin (0.75 microg/L) on antioxidants in a freshwater fish, Channa punctatus Bloch, using standard laboratory conditions. A single exposure for 48 h caused induction of various antioxidant enzymes and nonenzymatic antioxidants in kidney and liver. The induction of these antioxidants was not very prominent in gills. In fact, certain antioxidants were found to be depleted in gills. Catalase activity was decreased in all the tissues. Deltamethrin also induced lipid peroxidation in all the tissues, gills showing the highest levels. Glutathione, which is an established nonenzymatic antioxidant in fish, was significantly (P<0.001) increased in all the tissues. Ascorbic acid content increased in kidney and liver while it decreased in gills. The findings of the present investigation show that deltamethrin has oxidative-stress-inducing potential in fish, and gills are the most sensitive organs. It is also interesting to note that gills are the primary sites of deltamethrin absorption and their antioxidant potential is also very poor. The various parameters studied in this investigation can also be used as biomarkers of exposure to deltamethrin. It is suggested that appropriate ecotoxicological risk assessment should be made in the areas where deltamethrin is proposed to be used in pest control activities.