We have established a time course for the early morphological events of sporulation in Bacillus subtilis and related this to changes in gene expression, particularly those occurring in the prespore compartment. We have also systematically studied the effects of mutations in various regulatory (spo) genes on prespore-specific gene expression. On the basis of these results, and those of other laboratories, at least four distinct temporal classes of prespore-specific gene expression can now be distinguished. The first class begins within 15 min of the formation of the spore septum, and requires the sigma F form of RNA polymerase. The second class, also directed by RNA polymerase containing sigma F, begins soon after the completion of prespore engulfment, and depends on an intercellular signal from the mother cell. This transcription results in synthesis of sigma G. However, sigma G activity, directing the third class of gene expression, appears only about 30 min later and is dependent on the completion of prespore engulfment and on further interactions with the mother cell. The fourth class of gene expression has been described. The results demonstrate that the prespore programme of gene expression incorporates a series of control points modulated by information from the mother cell and on progress through the morphogenetic process.