Flexibacter columnaris-like bacterium (Strain NCMB 2158), isolated from Dover sole (Solea solea) suffering from "black patch necrosis" (BPN) in Scotland, was compared to F. columnaris, F. psychrophilus, F. maritimus, and to all other valid Flexibacter and Cytophaga species. Investigation of phenotypic characteristics showed the unidentified isolate to be more closely related to F. maritimus than to F. columnaris. DNA was subsequently extracted from each of the studied strains and compared using the DNA/DNA hybridization method. The very high DNA relatedness of Strain NCMB 2158 with 2 F. maritimus reference strains and its insignificant homology with all of the other strains confirmed that the Dover sole isolate belonged to the species F. maritimus. These results have resolved the bacterial aetiology of BPN and reveal for the first time the existence of F. maritimus elsewhere than in Japan.