During testing hatchery and wild fish from Ontario sources, we isolated bacteria that had sufficient characteristics in common with Y. ruckeri to suggest they were strains of this organism. Three isolates from brook trout (Salvelinus fontinalis), rainbow trout (Salmo gairdneri), and cisco (Corogonus artedii) biochemically resembled the sorbitol-fermenting Serovar II strain of Y. ruckeri, but gave cross reactions with antiserum to the Hagerman strain, Serovar 1. A fourth isolate from brown trout (Samo trutta) appeared to be different from the three previously described serovars. A fifth isolate previously obtained from muskrat (Ondatra ziberthica) intestine biochemically and serologically corresponded to the Hagerman serovar. We suggest that the geographic range of Y. ruckeri and variations in its biochemical and serological properties are greater than previously believed. These findings are of significance in diagnosis and regulation of enteric redmouth disease of salmonids.