Sporulation in Bacillus subtilis is a primitive differentiation process involving two cell types, the forespore and the mother cell. Each cell implements two successive transcription programmes controlled by specific sigma factors. We report that activity of σG, the late forespore sigma factor, is kept in check by Gin, the product of csfB, a gene controlled by σF, the early forespore sigma factor. Gin abolishes σG transcriptional activity when σG is artificially synthesized during growth, but has no effect on σF. Gin interacts strongly with σG but not with σF in a yeast two‐hybrid experiment. The absence of Gin allows σG to be active during sporulation independently of the mother‐cell development to which it is normally coupled. Premature σG activity leads to the formation of slow‐germinating spores, and complete deregulation of σG synthesis is lethal when combined with gin inactivation. Gin allows σF to delay the switch to the late forespore transcription programme by preventing σG to take over before the cell has reached a critical stage of development. A similar strategy, following a completely unrelated route, is used by the mother cell.