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Abstract. The effects of Bacillus thuringiensis (Bt) Cry1C toxin on the metabolic rate of Cry1C resistant and susceptible Spodoptera exigua (Hu¨bner) (Lepidoptera: Noctuidae) are investigated using closed-system respirometry. Mechanisms of resistance to the Bt toxin may be associated with an energetic cost that can be measured as an increase in metabolic rate compared with Bt-susceptible insects. This hypothesis is tested using third-and fifth-instar larvae and 1-7-day-old pupae. Metabolic rate is measured as the amount of O 2 consumed and CO 2 produced. ) of third-instar Cry1C resistant larvae reared continuously on a diet containing 320 mg Cry1C toxin per g diet (CryonT) are significantly greater than third-instar Cry1C resistant larvae reared on toxin for 5 days and reared thereafter on untreated diet (Cry5dT), Cry1C resistant larvae reared on untreated diet (CryReg) and the susceptible parental strain (SeA) reared on untreated diet. There are no differences in _ V V O 2 and _ V V CO 2 (mL g À1 h À1 ) among treatment groups for fifth-instar larvae. CryonT larvae and pupae weigh significantly less than larvae and pupae receiving other treatments. Smaller body mass may be an important biological cost to individuals exposed continuously to Bt toxin. One-day-old pupae of all treatment groups exhibit a high _ V V O 2 (mean approximately 0.174 mL g À1 h À1 ) with CryonT having a significantly greater value than all other treatments; there are no differences among the other treatments. Pupal metabolic rates of all treatment groups decline to a minimum between days 2 and 4 then increase linearly between days 4 and 7 until adult emergence. These results demonstrate no difference in metabolic rates, and possibly fitness costs, between resistant (CryReg and Cry5dT) and susceptible (SeA) S. exigua except when larvae were reared continuously on toxin (CryonT)