Currently, silkworm rearing with artificial diets through the larval stages is not popular because of the cost. Use of GM crops (e.g., Bt crops) as the ingredients of the artificial diets could decrease the cost of silkworm rearing. We studied the impact of Bt crops (Bt corn and Bt soybean) fed to the second instar larvae of the silkworm. MON 89034 (a Bt corn strain) had a potent insecticidal activity to the silkworm, but lost the activity after steam cooking for 60 min. Other corn strains (including a Non-GM and two Bt corn strains) had no insecticidal activity to the silkworm, but had growth inhibiting activity, which also lost the activity after the steam cooking. Both strains of soybeans (Non-GM and Bt soybean) had strong growth inhibiting activity to the silkworm, also lost the activity after steam cooking. In addition, we studied the toxicity of a commercial corn (USA product) after steam cooking for various times (3, 5, 10, 20, 40, 60, and 90 min). Three min steam-cooked sample had insecticidal activity and was positive for GM proteins (Cry1Ab, Cry3Bb, Cry1F, and Cry2A) by imunochromatography. However, 10 min steam-cooked sample had no insect growth inhibiting activity and lost the immunoreactive GM proteins. In conclusion, general cooking method of artificial diet including steam cooking process is enough for inactivation of toxic substances in Bt crops, suggesting the usefulness of these GM crops as the ingredients of the artificial diets of the silkworm. Institute of Agrobiological Sciences, National Agriculture and Food Research Organization, 1-2 Owashi, Tsukuba, Ibaraki, 305-8634, Japan