Type IV pili have been purified from strains of most of the Aeromonas species associated with gastroenteritis (A. veronii biovar sobria, A. hydrophila, A. trota and A. caviae). They appear to be a related family (molecular mass of pilin 19 to 23 kDa) with a tendency to bundle-formation. Hence, we have designated them 'bundle-forming pili' (Bfp). A type IV pilus biogenesis gene cluster (tapABCD) recently cloned from a strain of A. hydrophila, however, encoded a 17 kDa pilin which differed significantly in its N-terminal amino acid sequence from the Bfp pilins. This paper describes the cloning of part (tapA and ~20% of tapB) of a homologous pilin gene cluster from a Bfp-positive strain of A. veronii biovar sobria, and presents evidence that the entire pilin gene cluster (tapABCD) is present in this strain. The predicted N-terminal amino acid sequence of the pilin encoded by the A. veronii biovar sobria tapA differed markedly from the corresponding sequence of its Bfp pilin, and those of the Bfp purified from other Aeromonas strains and species. Probing with tapA and tapD genes showed that these Bfp-positive Aeromonas strains also possessed the Tap gene cluster. TapA proteins of A. veronii biovar sobria and A. hydrophila shared 53% identity and 63% homology. We conclude that Aeromonas species are potentially able to express at least two distinct families of type IV pili (Bfp and Tap).