A study of physiological and biochemical properties of 12 strains of Bacillus thuringiensis subsp. kurstaki was performed. The strains studied included natural isolates similar to the type strain in their properties, a Cry-Spo+ mutant, and also mutants assigned to C-1 or C-73 crystovar according to the properties of their parasporal crystals and the ability to hydrolyze esculin, generate acid from salicin, and exert urease activity. The crystals produced by C-1 crystovar cultures were three times larger than those formed by C-73 crystovar bacteria; their distinctive feature was pathogenicity for mosquito larvae, and their effect on other insects (gypsy moth and meadow moth) was on the average twofold greater than the effect of C-73 crystovar crystals. A study of the antibacterial action of delta-endotoxins of B. thuringiensis subsp. kurstaki allowed us to more clearly elucidate the reasons for the differences in biological activity of C-1 and C-73 crystovars. Comparative analysis of the data obtained confirmed that the effect of delta-endotoxins, formed per unit volume of the culture liquid, on insects and microorganisms is an important characteristic of B. thuringiensis strains. Of all the strains studied, the Cry-Spo+ mutant was the most potent producer of proteases and lecithinase.