The LexA-regulated sulA (sfiA) gene of Escherichia coli encodes an unstable protein which inhibits cell division. By determining the nucleotide sequences of the corresponding genes from the related bacteria Salmonella typhimurium, Enterobacter aerogenes and Serratia marcescens it was found that the regulatory region and the LexA binding site (SOS box) have been better conserved during evolution than the coding sequence. The N terminus of the SulA protein [amino acid (aa) residues 1-30] has diverged extensively during the evolution of Enterobacteriaceae, whereas the central region (aa residues 31-149) has been well conserved. At the C terminus a sequence showing some homology to the N protein of phage lambda was detected that may represent a recognition site for the Lon protease, which is known to degrade both polypeptides. When expressed in E. coli, the foreign sulA genes did not block cell division suggesting that their products are inactive. This may indicate that the N terminus of the SulA protein is involved in recognizing the cell division apparatus.