rpoS homologues were identified in several Erwinia species using Escherichia coli rpoS sequences as probes. The rpoS gene from Erwinia carotovora was cloned and the deduced amino acid sequence had 91% identity to E. coli RpoS. The latter sigma factor regulates the stationary phase inducible HPII catalase activity of E. coli. In an E. coli rpoS mutant, the E. carotovora rpoS gene was also able to regulate synthesis of this catalase. The presence of a similar catalase in E. carotovora suggests that the structural gene for this may be part of the rpoS 'regulon' in Erwinia also. This study also showed that there are several differences in the gene organization of the rpoS region of the E. coli and E. carotovora chromosomes.