"Atypical", cytochrome oxidase-negative variants of the fish pathogen Aeromonas salmonicida, isolated from ulcerated flounder (Platichthys flesus), were studied using different methods. Two of the strains possessed a protein that corresponded to the A-layer protein of A. salmonicida. The strains reacted with antibodies against the A-layer and monoclonal antibodies against the O-antigen of typical A. salmonicida. These tests confirm that the isolates from flounder should be classified as A. salmonicida. Analysis of the fatty acids showed that the isolates were rather homogenous but the values of the guanine plus cytosine content of the DNA of the bacteria varied too much for any conclusion to be drawn on their taxonomic location. The strains examined exhibited several biochemical characters that differed from those of the type strains of A. salmonicida subsp. salmonicida and A. salmonicida subsp. achromogenes. The results suggest that these "atypical", cytochrome oxidase-negative variants may form a new subspecies of A. salmonicida.