whiI is one of several loci originally described as essential for sporulation in Streptomyces coelicolor A3(2). We have characterized whiI at the molecular level. It encodes an atypical member of the response regulator family of proteins, lacking at least two of the residues strongly conserved in the conventional phosphorylation pocket. It is not adjacent to a potential sensor kinase gene. Fifteen mutant alleles of whiI were sequenced, revealing, among others, six mutations affecting conserved amino acids, several frameshift mutations and one mutation in the promoter. The whiI promoter is specifically transcribed by the sporulation-specific σWhiG-containing form of RNA polymerase. Transcription of whiI is temporally controlled, reaching a maximum level coincident with the formation of spores. Further transcriptional studies suggested that WhiI is involved directly or indirectly in repressing its own expression and that of another σWhiG-dependent sporulation-specific regulatory gene, whiH.