Transformation of rice with genes encoding insecticidal Cry proteins from Bacillus thuringiensis (Bt) should confer high resistance to target lepidopteran pests, such as Chilo suppressalis, and low toxicity to non-target organisms, such as silkworm Bombyx mori. Five purified Cry proteins that have been used for plant transformation were tested using dietary exposure assays. The susceptibility of C. suppressalis larvae to the five insecticidal proteins in the decreasing order was: Cry1Ca > Cry1Ab > Cry1Ac > Cry2Aa > Cry1Fa. However, the toxicities of the Cry proteins to B. mori were in the order: Cry1Fa > Cry1Ca > Cry2Aa > Cry1Ab > Cry1Ac. The Cry1Ca, Cry1Ab and Cry1Ac proteins exhibited relatively high toxicity to C. suppressalis larvae, with EC50 values of 16.4, 45.8 and 89.6 ng/g, respectively. The toxicities of the three Cry proteins to B. mori larvae were 8, 14, and 22 times lower, with EC50 values of 138.3, 628.4 and 1939.2 ng/g, respectively. The Cry1Fa and Cry2Aa proteins showed high toxicity to B. mori larvae, with EC50 values of 135.7 and 373.9 ng/g, respectively, but low toxicity to C. suppressalis larvae, with EC50 values of 6092.1 and 1208.5 ng/g, respectively. We thus conclude that Cry1Ab, Cry1Ac and Cry1Ca are appropriate for transforming rice to control lepidopteran rice pests. In contrast, Cry1Fa and Cry2Aa are not appropriate due to their high toxicity to silkworm larvae and low activity against the target pest.