Antioxidant defense components protect insects by scavenging reactive oxygen species, leading to oxidative stress. I therefore investigated the effects of an organophosphorous insecticide, malathion, on superoxide dismutase (SOD) and acetylcholinesterase (AChE) activities as well as glutathione (GSH) and malondialdehyde (MDA) content as oxidative stress biomarkers in whole body of greater wax moth, Galleria mellonella (L.), larvae. Subcellular fractionation also was assayed for SOD and AChE enzymes to assess subcellular toxicity of malathion in this wax moth. The newly hatched larvae were reared on diets containing 0.01, 0.1, 1.0, and 10 ppm malathion. The diet with lowest concentration of malathion did not significantly influence MDA content and AChE activity. Malathion at 1.0 ppm significantly resulted in increased MDA content and decreased AChE activity. I observed a significant increase in SOD activity, whereas total GSH content and AChE activity were significantly lower for 1.0 ppm malathion than the control groups. Highest concentration of dietary malathion significantly decreased SOD and AChE activities, and GSH content in whole body of the insect. Subcellular fractionations showed that activities of microsomal and soluble AChE, and microsomal SOD for high concentrations of malathion (1.0 and 10 ppm) were significantly lower than control. Soluble SOD activities were significantly increased by low malathion concentrations, whereas only the highest malathion concentration resulted in significantly decreased SOD activity. I infer that induction of antioxidant defense mechanisms in response to increased oxidative stress may be a result of AChE inhibition by malathion in G. mellonella larvae.