The spoIIAB gene of Bacillus subtilis encodes an inhibitor of sigma F, a transcription factor that plays a crucial role in the establishment of prespore-specific gene expression during sporulation. The SpoIIAB protein can probably also inhibit a closely related sigma factor sigma G, which determines the later phase of prespore-specific transcription. We have isolated two new missense mutations in the spoIIAB gene. spoIIAB28 behaves like the previously described spoIIAB1 mutation, in that it mainly affects the activity of sigma G. In contrast, the spoIIAB22 mutation seems to be impaired mainly in its ability to inhibit sigma F. All three missense mutations are clustered in the N-terminal coding region of spoIIAB, suggesting that this region of the protein interacts with the sigma factors. The extreme N-terminal part of SpoIIAB may be specifically concerned with the regulation of sigma G activity.