Geographic variation in the susceptibility of the striped stem borer, Chilo suppressalis (Walker) (Lepidoptera: Pyralidae), in China to Bacillus thuringiensis (Bt) insecticidal crystal proteins Cry1Ac and Cry1Ab was studied to establish baseline information for comparing the future response of populations with increased exposure to Bt products. Rice is the major host of C. suppressalis, and Bt rice may be released in China in the near future. Twelve populations of the pest were collected from the major rice-growing regions of China. LC₅₀ estimates were determined for all populations for Cry1Ac and for eight populations for Cry1Ab. The bioassay results indicated that the range of LC₅₀ in neonate larvae to Cry1Ac and Cry1Ab was from ≈15 to ≈157 mg (AI)/L and ≈2 to ≈34 mg (AI)/L, respectively. LC₅₀ values were lower for Cry1Ab than for Cry1Ac, and there was a significant positive correlation between the two toxins tested.