The spore coat of Bacillus subtilis has a unique morphology and consists of polypeptides of different sizes, whose synthesis and assembly are precisely regulated by a cascade of transcription factors and regulatory proteins. We examined the factors that regulate cotS gene expression and CotS assembly into the coat layer of B. subtilis by Northern blot and Western blot analysis. Transcription of cotS mRNA was not detected in sporulating cells of ςK and gerE mutants by Northern blot analysis. By Western blot analysis using anti-CotS antibody, CotS was first detected in protein samples solubilized from wild-type cells at 5 h after the start of sporulation. CotS was not detected in the vegetative cells and spores of a gerEmutant or in the spores of mutants deficient in ςE, ςF, ςG, or ςK. CotS was detected in the sporangium but not in the spores of a cotEmutant. The sequence of the promoter region of cotS was similar to the consensus sequences for binding of ςK and GerE. These results demonstrate that ςK and GerE are required for cotS expression and that CotE is essential for the assembly of CotS in the coat. Immunoelectron microscopic observation using anti-CotS antibody revealed that CotS is located within the spore coat, in particular in the inner coats of dormant spores.