The colonization mechanisms of enteropathogenic Aeromonas strains are poorly characterized, but recent studies indicate that some filamentous structures are intestinal adhesins. This study describes the purification and characterization of a long, flexible pilus from a gastroenteritis-associated strain of Aeromonas veronii biovar sobria. SDS-PAGE analysis (various conditions) of pili preparations yielded a pilin protein band of approximately 21 kDa. Its N-terminal amino acid sequence was unambiguous and homologous with those of type IV pilins. Immunogold electron microscopy with rabbit antisera produced against this pilin protein (SFP) decorated single pili and rope-like bundles of pili on the bacterial surface. These were seen more frequently on strains grown at 22 degrees C compared with 37 degrees C and in liquid rather than on solid medium. SFP was not detected on any of 104 strains of Aeromonas (different species and sources) from our culture collection, although morphologically similar structures were seen on a number of these strains. This finding and differences among other published amino acid sequences show that Aeromonas type IV pili are antigenically diverse. Bundle-forming type IV 'class B' pili are important in the virulence of other enteropathogenic bacteria. The N-terminal amino acid sequence of the Aeromonas SFP, however, showed closer homology to the type IV 'class A' pilins. Studies are in progress to investigate the role of SFP in Aeromonas virulence.