Bacillus thuringiensis is one of the most widely used sources of biorational pesticides, as well as a key source of genes for transgenic expression to provide pest resistance in plants. In this study the effect of Bacillus thuringiensis ssp. galleriae (Bt) infection on the activity of superoxide dismutase (SOD), glutathione S-transferase (GST), catalase (CAT), concentrations of oxidated and reduced thiols (RSSR/RSH) and malondialdehyde (MDA) was tested in the midgut of Galleria mellonella larvae. We found that Bt infection resulted in increased activities of SOD, GST, malondialdehyde and RSSR/RSH ratio the first day after inoculation. However, catalase activity decreased on the first and following days after bacterial infection by Bt. Our results confirm the hypothesis that Bt infection increases the level of oxidative stress in the larval midgut. In light of this study, it seems possible that oxidative damage contributes to cell death in the midgut during bacteriosis.