We have isolated a sigma-like regulatory protein termed sigma 29 whose synthesis or association with Bacillus subtilis RNA polymerase was induced during spore formation. sigma 29 is a sporulation-specific component of RNA polymerase as it was absent in enzyme from an early-blocked sporulation mutant (SpoOA). We have demonstrated specific RNA synthesis by sigma 29-RNA polymerase using as a DNA template a cloned cluster of vegetative and sporulation genes from the purA-cysA region of the B. subtilis chromosome. The pattern of gene recognition by sigma 29-RNA polymerase was distinct from that observed for RNA polymerases containing sigma 55 or sigma 37, species of sigma factor that are present in vegetative cells of B. subtilis. A reconstitution experiment in which purified sigma 29 was added to core RNA polymerase demonstrates that sigma 29 was directly responsible for the altered transcriptional specificity of sporulation RNA polymerase. We propose that sigma 29 is a regulatory protein that controls developmental gene transcription at an early stage of spore formation.