Yersinia ruckeri causes salmonid fish diseases called yersiniosis or enteric redmouth disease (ERM). Isolates include several serological varieties and disease outbreaks are frequently associated with stress or poor environmental factors. As a result, it is difficult to define clearly the significant virulence factors and pathogenic mechanisms of the bacterium, which introduces uncertainties about the appropriate formulation of bacterins for immunization. An enteric redmouth bacterin was the first commercially-produced fish vaccine, and the formalin-killed whole-cell product continues to be highly effective whether administered by immersion, spray, injection, or oral routes. Serovar 1, "Hagerman" strains are the basis for most commercial bacterins, and serovar 2 is not included, despite epizootics in chinook salmon and brook trout. Vaccination studies report different degrees of cross-protection between serogroups of Y. ruckeri, but the basis for the cross-protection is not clear. Lipopolysaccharide (LPS) of serovar 1 Y. ruckeri elicits negligible or weak antibody responses in fish and low cell-proliferation memory responses compared with serovar 2 strains. These observations raise fundamental questions about the kinds of immune responses that are involved in the highly effective vaccine-protection provided by commercial vaccines.