Abstract.The role of the caseinase extracellular enzyme in the pathogenesis of furunculosis was investigated. Natural and mutant strains ofAeromonas salmonicidawhich were unable, or limited, in their ability to produce this enzyme were studied. The virulence of the mutant strains ofA. salmonicidawas not significantly reduced by the lack of extracellular caseinase production. Furthermore, there was no difference in the toxicity of extracellular products produced by these mutant strains and the wild‐typeA. salmonicidastrain studied. It is reasonable to assume from these results that the caseinase enzyme has no major role in the pathogenesis ofA. salmonicidainfection