Geographic variability in susceptibility of field-collected Cnaphalocrocis medinalis (Guenée) (Lepidoptera: Pyralidae) to Bacillus thuringiensis (Berliner) was studied to establish a geographic baseline for comparison of future population responses to the increased use of B. thuringiensis-based insect control products. Colonies of C. medinalis were established from 10 populations collected from 10 different provinces in the major rice (Oryza savita L.)-growing regions of China. Populations were evaluated for their susceptibility to Cry1Ac and Cry1Ab endotoxin proteins with the leaf-dip bioassay. The LC₅₀ values to Cry1Ac and Cry1Ab were estimated for the 10 populations. Bioassay results indicated that the ranges of LC₅₀ in the second instars of C. medinalis to Cry1Ac and Cry1Ab were from 3.77 to 208.22 mg ([AI])/liter and 0.22–7.05 mg ([AI])/liter, respectively. The relative ratios in susceptibility between the most susceptible and the most tolerant populations were beyond 50-fold for Cry1Ac and 30-fold for Cry1Ab. Moreover, there was a significant positive correlation between susceptibilities to the two toxins tested, suggesting that insect populations that are relatively tolerant to one protein are also relatively tolerant to the other.