The spores of crystal-forming (Cry(+)) and non-crystal-forming (Cry(-)) strains of Bacillus thuringiensis var. kurstaki and Bacillus cereus were tested for the ability to be activated by 0.1 M K2CO3 (pH 10). Only the spores of crystal-forming strains could be activated, and this phenotype was independent of whether crystals were present with the spores in the activation solution, The spores of a B. thuringiensis var. kurstaki strain that is temperature sensitive for protoxin accumulation could be activated by the alkaline solution when produced at the permissive temperature, whereas spores produced at the nonpermissive temperature were not activated. The results indicate that protoxin in the spore coat is responsible for the alkaline-activation phenotype and may serve an ecological function for the organism.