Diversity within and around the flagellin (fla) A gene of Helicobacter pylori was studied by polymerase chain reaction/restriction fragment length polymorphism (PCR/RFLP) analysis and genomic Southern blot hybridization profiling. Four distinct pattern types were identified by DdeI restriction analysis of the 1.5-kb flaA amplicon of 55 strains. Most strains (73%) had the same flaA RFLP type, but subtypic variation was evident in some strains. No consistent associations were observed for selected strain subsets between the DdeI flaA profiles and phenotype (motility and cytotoxicity), urease gene profile or patient symptomatology. A subset of seven (F-1 profile) and four (F-2 profile) strains with identical HindIII digest patterns provided further evidence that the flaA gene was relatively highly conserved within H. pylori. By contrast, the flaA gene blot hybridization profiles were more diverse and consistent with greater variation at restriction sites in adjacent regions of the genome. We conclude that analyses of polymorphisms within the flaA gene provide limited discrimination between strains of H. pylori. The flaA genomic blot profiles offer greater potential for molecular typing purposes, although no associations with other pathogenicity factors or disease symptoms could be deduced.