The spiral microaerophilic bacterium Helicobacter mustelae is linked to gastritis and gastric ulcers in ferrets. Electron microscopy of H. mustelae showed the presence of a laterally extensive array of 8.5-nm-diameter rings on the cell surface, which was shown to be composed of a 150 kDa protein. This protein was purified, and the sequence of 10 amino-terminal residues was determined. Polyclonal antibody against the purified 150 kDa protein labelled the ring structures on the homologous strain by means of immunogold. Cross-reactive proteins were identified in three H. mustelae strains, but not in Helicobacter pylori or Helicobacter felis. The hsr gene encoding this protein was cloned, and the protein expressed in Escherichia coli independently of vector promoters. The 1519-codon nucleotide sequence of the gene was determined, and comparison with the chemically derived protein sequence indicated a 47-residue leader peptide, and a mature protein with a molecular weight of 152300. Thus the cell surface of H. mustelae differs markedly from other members of the genus Helicobacter in being covered by an array of 8.5 nm rings composed of a 150 kDa protein.