Flavobacterium columnare is a thin Gram-negative rod bacterium of the genus Flavobacterium. The name derives from the way in which the organism grows in rhizoid columnar formations. The species was first described by Davis (1922), and the name was validated by Bernardet and Grimont (1989). Flavobacterium columnare can be identified in the laboratory by a five-step method that demonstrates: The species has been known previously as Flexibacter columnaris, Bacillus columnaris, and Cytophaga columnaris. Flavobacterium columnare is one of the oldest known diseases among warm-water fish, and manifests itself as an infection commonly known as columnaris. Infections are the second leading cause of mortality in pond raised catfish in the southeastern United States. Early treatment with potassium permanganate has been shown to increase survival rate, although the difference was not statistically significant.