Pagrosomus major larvae were exposed to thewater-soluble fraction of hydrocarbon in No.0 diesel oil(corresponding to No. 2 fuel oil) at concentrations of 0, 0.l7,l.22 and 8.82 mg/L for up to l5 days. Larvae were sampled on days9 and l5 of the experiment. Supernatants of viscera tissueextractions were assayed for biochemical response in terms ofoxidative stress-superoxide dismutase(SOD), activity ofselenium-dependant glutathione peroxidase(Se-GPx) and catalase(Ca), and the concentration of reduced glutathione(GSH). On day 9of exposure, statistically significant dose-related increases inSe-GPx and SOD activity, and GSH concentration were observed in allcases except for Se-GPx activity under the highest dosage of hydrocarbon. However, on day l5 of exposure, a similar dose-relatedresponse was only observed for Se-GPx activity. GSH concentrationdecreased and SOD activity showed no statistical difference ascompared to controls. However, a significant decrease in comparedto day 9 Se-GPx activity and GSH concentration, in contrast toincreased SOD activity at day 15 as indicates an acceleratedaccumulation of H2O2 and potential oxidative damage under long termexposure of larvae to hydrocarbons. No statistical changes wereobserved in Ca activity throughout the experiment, possibly owingto the high efficiency of Se-GPx.A recovery experiment was performed on indicating that theresponse of antioxidants measured tending to return to theircontrol levels. These results prove the function of the antioxidantdefense system of the larvae to the water-soluble fraction ofhydrocarbons in No. 0 diesel oil.