Aeromonas salmonicida subsp. salmonicida was originally described agent of furunculosis in salmonid fish at the end of 19th century, while atypical A. salmonicida has been isolated from a wide range of cultivated and wild fish species, non-salmonids as well as salmonids, inhabiting fresh water, brackish water and marine environments. Although A. salmonicida subsp. salmonicida is homogeneous, atypical strains are more heterogeneous with respect to biochemical characteristics and growth conditions. Based on the nucleotide sequences of 16S rDNA, Japanese isolates of atypical A. salmoicida from different fish species can be divided into four groups. This review describes the outlines and the topics of the diseases.
