The present knowledge concerning the virulence mechanisms of Cytophaga psychrophila and other Cytophaga-like pathogens for fish, including Cytophaga columnaris, Flexibacter maritimus, and Flexibacter ovolyticus is reviewed as is the taxonomic work defining the genera in the Cytophaga-Flexibacter phylogenetic branch. Although specific adhesins have not been described, many Cytophaga-like bacteria are surrounded by a thin slime layer that attaches the cells to a surface but allows their translational moves as well. Many Cytophaga-like bacteria also exhibit bacteriolytic activity, which seems to be a characteristic of the order Cytophagales, however these activities are not useful in distinguishing pathogenic from nonpathogenic strains. Extracellular proteases produced by Cytophaga columnaris and Cytophaga psychrophila have been identified as have a protease, hemolysin, LPS, and extracellular products (ECP) in Flexibacter maritimus with the ECP and hemolysin possibly being related to its pathogenicity. Four different plasmid profiles have been demonstrated in Cytophaga psychrophila but at present their role, if any, in virulence is not known. In Cytophaga psychrophila there appears to be a common species-specific antigen, and somatic antigen may be used to distinguish pathogenic from nonpathogenic strains. In spite of the importance of Cytophaga psychrophila, its pathogenicity is still poorly understood.