Sporulation of Bacillus subtilis involves the differentiation of two cell types, the mother cell and the forespore. Two key regulators of mother-cell gene expression are SpoIIID, a DNA-binding protein that activates or represses transcription of many different genes, and sigma K, a subunit of RNA polymerase that directs the enzyme to transcribe genes encoding proteins that form the spore coat. Previous studies showed that SpoIIID is needed to produce sigma K, but suggested that SpoIIID represses sigma K-directed transcription of genes encoding spore coat proteins. Here we show that a feedback loop connects the levels of sigma K and SpoIIID, such that production of sigma K leads to a decrease in the level of SpoIIID. The existence of the feedback loop was demonstrated by using antibodies prepared against SpoIIID to measure the level of SpoIIID during sporulation of wild-type cells, mutants defective in sigma K production, and a mutant engineered to produce sigma K earlier than normal. The feedback loop operates at the level of synthesis and/or stability of spoIIID mRNA, as demonstrated by measuring the level of spoIIID mRNA during sporulation of wild-type cells and mutants defective in sigma K production. Our results suggest that a rise in the level of sigma K during the stage (IV) of spore cortex formation causes a decrease in the level of SpoIIID, which, at least in part, establishes the switch to the stage V (spore coat formation) pattern of mother-cell gene expression.