Fifty two isolates of atypical Aeromonas salmonicida, recovered from a wide range of hosts and geographical locations, were heterogeneous in terms of molecular and phenotypic characteristics, and represented taxa which could not be accommodated by the current classification of four subspecies. Generally, there was incongruence between the molecular (PCR, RAPD and ribotyping) and phenotypic methods in terms of cluster membership. By PCR, 6 groups were described of which Group 1 encompassed 12 isolates including the type strain of A. salmonicida subsp. smithia. Group 2 accommodated 23 isolates including the reference cultures of subspecies achromogenes and masoucida. The named culture of Haemophilus piscium was recovered in Group 6. By ribotyping and RAPD, the reference cultures were recovered in separate groups. All methods pointed to the uniqueness of subspecies smithia. Most isolates contained 2-6 plasmids, of 2.3 to 150 kb in length. Nevertheless, all isolates possessed certain key characteristics, including Gram-negativity, and the absence of motility.