Infections with motile Aeromonas species were detected in lake whitefish collected over a one-year period from four stocks within lakes Michigan and Huron, USA. Sixty-nine isolates were recovered from the kidneys and swim-bladders of sixty-four infected fish. Representative isolates were Gram-negative bacilli that produced cytochrome oxidase, grew in the absence of salt, were facultative anaerobes, and were resistant to the vibriostatic agent 2,4-diamino,6,7-di-isopropylpteridine. Phenotypic characterization placed twenty-two isolates into the A. hydrophila complex, twelve into the A. sobria complex, and one into the A. caviae complex, while six isolates were characterized as A. allosaccharophila, two as A. veronii bv. veronii, and one as A. popoffi. The prevalence of infection by motile aeromonads varied by site and season, with lake whitefish sampled in the summer having a significantly higher prevalence. Clinical signs in lake whitefish infected only with Aeromonas spp. included congestion and hemorrhaging in the fins and musculature; generalized pallor; congestion, hemorrhaging, and multifocal necrotic foci within the liver; moderate to severe splenomegaly; congestion and swelling of the kidneys; ascites within the peritoneal cavity; and hemorrhagic enteritis. This study provides evidence on the wide spread prevalence of motile aeromonad infections in lake whitefish stocks inhabiting northern lakes Michigan and Huron.