The spo-279(ts) mutation, originally thought to be located in the spoIIG operon of Bacillus subtilis, has been mapped in close proximity but outside of the spoIIG locus. This mutation defines a new gene, spoIIN, located midway between the spoIIG and the spoVE loci, and whose product is required for successful completion of the asymmetric septation step. The spoIIN locus was cloned using a combination of 'walking steps' upstream from the spoIIG region and hybridization screening of a bacteriophage lambda library. Sequencing of DNA fragments able to rescue the spoIIN279(ts) mutation revealed that the spoIIN locus is identical with the B subtilis counterpart of the Escherichia coli ftsA gene. After cloning the ftsA region from a strain containing the spoIIN279(ts) mutation we found that this mutation converts the ninth residue of the FtsA protein from serine to asparagine. The spoIIN279(ts) mutation, which is recessive, leads to filamentation during growth at 42 degrees C and causes defective formation of the sporulation septum at this non-permissive temperature. The FtsA protein is therefore required for proper cell septation, both during vegetative growth and sporulation. Possible additional roles of FtsA during sporulation are discussed.