Bacillus sphaericus ts-D1290 lethal mutant was comparatively characterized with the wild type strain 1593 by the measurements of the biosynthesis of total proteins on the temperature-sensitive cultures at permissive temperature of 30℃ and at nonpermissive temperature of 42℃. The protein syntheses in the spores of the ts-D1290 and the wild type strain at 30℃ occurred in 6 to 8 hours after inoculation. When the spores of the mutant were cultured at 42℃, they germinated, but the protein synthesis almost did not occur, and the number of viable cells did not increase at 42℃, Vegitative cells after the germination of the wild type spores at 30℃. proteins of 140 and 150 kilodaltons produced within 1 or 2 hours in the cells; however, after a 2 h culture, the amounts of the 150 Kd protein increased. While the 140Kd protein did not produced in the mutant, only the 150 Kd protein produced in the cell. The 155 Kd protein produced after a 4 h culture at 30℃.