A mutation in any one of eight spoO genes of Bacillus subtilis blocks the process of spore formation at its earliest stage. To investigate how the products of the spoO genes may be involved in developmental gene expression, we fused the lacZ gene of E. coli to spoVG, a sporulation gene whose induction at the onset of sporulation is under spoO control. In cells of Spo+ bacteria containing a single copy of the gene fusion, conditions leading to the onset of sporulation resulted in the induction of beta-galactosidase synthesis. This induction was moderately to severely impaired by mutations in any of seven spoO genes. Deletion and hybridization analysis demonstrated that this sporulation-induced enzyme synthesis was exclusively expressed from the two overlapping promoters, which comprise the spoVG transcription-initiation region, and that a small DNA segment (157 bp) containing the spoVG promoters was sufficient to cause spoO-dependent induction of the fused lacZ gene.