Lymantria dispar larvae are very hazardous to forests and they also cause allergic reactions in humans. In the present study, distinct concentrations of methidathion were applied to L. dispar larvae via oak leaves which were prepared by the dipping method. The data obtained was statistically appraised using probit analysis and a LC50/48h value for L. dispar larvae found to be 25,480 ppm. Antioxidant defense components protect insects by scavenging reactive oxygen species, leading to oxidative stress. The present study was investigated the effects of LC50/48h value of methidathion, on the oxidative stress indicator, malondialdehyde (MDA), and antioxidant enzymes [superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx)] activities in midgut tissues. There were statistically significant increases in the MDA level and SOD, CAT, GPx activities in the LC50/48h concentration of methidathion-treated larvae compared to the control larvae. These results indicated that methidathion which is an organophosphate pesticide, causes an increase in oxidative stress and we infered that increased oxidative stress induces antioxidant defence mechanisms.