The gene encoding a 29-kDa flagellar sheath protein was cloned and found to be similar to hpaA, reported to encode an N-acetylneuraminyllactose-binding fibrillar hemagglutinin (D. G. Evans, T. K. Karjalainen, D. J. Evans, Jr., D. Y. Graham, and C. H. Lee, J. Bacteriol. 175:674-683, 1993). The transcriptional start was mapped by primer extension from Helicobacter pylori mRNA, indicating an active consensus promoter at a location different from that suggested by Evans et al. Immunogold labelling of the flagellar sheath with a monoclonal antibody to HpaA was demonstrated in four strains, contrary to previous reports of a surface (D. G. Evans, T. K. Karjalainen, D. J. Evans, Jr., D. Y. Graham, and C. H. Lee, J. Bacteriol. 175:674-683, 1993) or a cytoplasmic (P. W. O'Toole, L. Janzon, P. Doig, J. Huang, M. Kostrzynska, and T. J. Trust, J. Bacteriol. 177:6049-6057, 1995) locale. Agglutination of erythrocytes and adherence to AGS cells by a delta hpaA mutant were no different from those of the parent strain, confirming a recent finding of O'Toole et al.