ABSTRACT: Atypical strains of Aeromonas salmonicida are the causal agent of atypical furunculosis or ulcer disease in various fish species, including spotted wolffish Anarhichas minor, which is a promising species in the Norwegian fish-farming industry. Isolates of atypical A. salmonicida comprise a very heterogenous group showing large variety in biochemical, molecular and virulence characteristics. The genetic variability among atypical isolates from wolffish was characterised using amplified fragment length polymorphism analysis: AFLP-fingerprinting. Additional isolates from halibut Hippoglossus hippoglossus, turbot Scophthalmus maximus, cod Gadus morhua and several salmonid fishes were included for assessment of variability and relatedness among a total of 56 atypical isolates of A. salmonicida. They were compared to reference strains of A. salmonicida subspecies and to other Aeromonas species pathogenic in fishes. AFLP-fingerprints subjected to similarity analysis yielded a grouping of the isolates into several clusters, revealing genetic heterogeneity among the isolates. There seems to be a correlation between genetic similarity among isolates and the fish host. The Icelandic isolates, mainly from cod, formed a very homogeneous subcluster, which was closely related to the wolffish isolates. All atypical isolates from spotted and common wolffish grouped together in a large cluster and appear to be very homogeneous, even though they had been isolated over a period of 8 yr at different locations in Norway. On the other hand, most of the isolates from turbot and halibut grouped together into 2 different clusters, while the 9 atypical isolates from salmonids appeared in 4 different clusters. Thus, the atypical isolates of A. salmonicida from halibut, turbot and salmonid fishes seem to be more genetically diverse than those from wolffish and cod.