The rpoS nucleotide and predicted amino acid sequences from three Escherichia coli O157:H7 isolates were compared with those from three other E. coli isolates, including the likely O157:H7 progenitor, E. coli O55:H7. These clinical and environmental isolates all had identical σS amino acid sequences, while laboratory strains K12 and DH1 had three and one amino acid alterations, respectively, in comparison with the majority sequence. To extend the analysis of σS sequence conservation to include other Gram-negative bacteria, the E. coliσS sequences were compared with those from diverse Gram-negative organisms; σS sequence identities ranged from 50·2 to 99·7% among the available sequences. The results further confirm the existence of rpoS alleles among different E. coli strains, although all strains were classified as acid-resistant with survival rates >10% after 2 h exposure to pH 2·5. It was also found that all E. coli O157:H7 isolates tested had a unique nucleotide at position 543, thus differentiating these strains from other E. coli serotypes.