Sporulation-specific mutations in Bacillus subtilis have been mapped by transduction and transformation. The mutations caused blocks at stages 0, II, III, and IV of sporulation; more than one phenotype was found for each of these stages. On the basis of the criteria used to define a sporulation operon, a minimum estimate could be made of the number of operons activated during sporulation. Nine operons were identified for stage 0, eight for stage II, five for stage III, and six for stage IV. It is probable that several of these 28 operons are activated in groups so that the number of steps in the dependent sequence of sporulation events should turn out to be less than the number of operons.