An oxidase-negative Aeromonas salmonicida was isolated from coho salmon (Oncorhynchus kisutch) suffering from an epizootic of furunculosis at the state hatchery near Belfair, Washington. Typical, oxidase-positive A. salmonicida was isolated concurrently from the same population of fish. Mortality was controlled with medicated feed treatments. Evidence supporting the identification of the two types of A. salmonicida is presented. Methods for the proper identification of oxidase-negative A. salmonicida isolates are evaluated.