An epizootic among perch (Perca fluviatilis L.) occurred during the summer and fall of 1979 in an alpine lake (Lake Annecy, Haute-Savoie) in France. Hemorrhagic and ulcerative clinical signs were associated with the height of the mortality. A Gram-negative, non-motile, slow-growing bacterium was isolated from skin lesions in diseased fish. Aeromonas hydrophila often was present. Since the non-motile bacterium was suspected to be the etiological agent, its characteristics and pathogenicity were determined. The bacterium was pathogenic to perch, possibly pathogenic to rainbow trout, and non-pathogenic to carp. It could be reisolated from infected fish, but its physiologic and biochemical properties have not been assessed to determine the taxonomic position.