[Aim]  To measure changes in the activity of protective and detoxifying enzymes in the F₀ and the F₁ generations of the 3rd instar of Riptortus pedestris nymphs exposed to different concentrations of thiamethoxam. [Methods]  Reniform soybean seeds soaked in water were immersed in 25% thiamethoxam WG that had been diluted with different amounts of water to obtain LC₁₀ (5.2 mg/L), LC₂₀ (11.2 mg/L), LC₃₀ (19.6 mg/L), LC₄₀ (31.6 mg/L) and LC₅₀ (49.4 mg/L) solutions. Water was the control. F₀ nymphs of R. pedestris were fed the treated seeds for 24 hours after which changes in the activity of peroxidase (POD), catalase (CAT), superoxide dismutase (SOD), acetylcholinesterase (AChE), glutathione S-transferase (GST) and carboxylesterase (CarE) in surviving nymphs was measured. Another group of nymphs was placed in cages and fed soybean seeds treated with water until F₁ generation nymphs had developed to the 3rd instar, after which changes in the activity of POD, CAT, SOD, AChE, GST, and CarE were determined in vivo. [Results]  After treatment with different concentrations of thiamethoxam, the POD, CAT, SOD, GST and CarE activity of F₀ generation nymphs was higher than that of the control group. POD, AChE, GST and CarE activity of F₁ larvae were also higher than that of the control group. The POD, GST and CarE activity of F₁ larvae was higher than those of F₀ larvae for all treatment concentrations. AChE enzyme activity of F₁ larvae was higher than that of F₀ larvae for concentrations in the range of LC₂₀-LC₅₀. Protective and detoxifying enzyme activity in F₀ and F₁ larvae was positively correlated with thiamethoxam concentration. [Conclusion]  F₀ R. pedestris nymphs detoxify thiamethoxam by increasing their POD, CAT, SOD, GST and CarE enzyme activity, whereas F₁ nymphs detoxify thiamethoxam by increasing their POD, AChE, GST and CarE activity. The observed changes in POD, AChE, GST and CarE enzyme activity may be the mechanism responsible for pesticide resistance in R. pedestris F₁ nymphs.