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Aeromonas hydrophila and other motile aeromonads are among the most common bacteria in freshwater habitats throughout the world, and are frequently associated with severe disease among cultured and feral fishes. Determinations of the etiology of diseases involving aeromonad infections has been complicated by the genetic, biochemical, and antigenic heterogeneity of members of this group. Consequently, motile aeromonads consist of a complex of disease organisms that are associated with bacterial hemorrhagic septicemias in fishes. From descriptions of fish diseases in the early scientific literature, Otte (1963) speculated that septicemic infections in fish caused by motile aeromonads were common throughout Europe during the Middle Ages. The bacterial etiology of these early reports was often inconclusive; however, the pathology described was analogous to that of red leg disease in frogs, in which A. hydrophila was also implicated as the causal organism.