Vaccination is a keystone in prophylactic strategies preventing outbreaks of fish pathogenic bacterial diseases in aquaculture. The first commercial fish vaccine consisted of a bacterin of Yersinia ruckeri serotype O1 biotype 1. The vaccine has been very successful and has been used for more than 35 years. A vast experience has been gained concerning the applications of the vaccine, which can be utilized through several mucosal immunization routes such as bath, oral and anal application, all resulting in significantly increased survival compared to un-vaccinated control groups during bath challenge experiments. Little is known regarding the nature of the protective immune response. We have shown that even low titers of Y. ruckeri specific antibodies confer protective immunity in vivo, and shown that is possible to pass immunity to naïve fish by passing serum containing Y. ruckeri specific antibodies. Further, plasma from bath vaccinated fish kills significantly more Y. ruckeri in vitro than plasma from un-vaccinated control fish. Increased plasma antibody titer against Y. ruckeri seems to be an important part of the protective immune response obtained post bath vaccination. These results all point towards an important role of specific antibodies as part of a vaccine-induced protective mechanism. However, we have shown full protection in both orally and anally immunized rainbow trout without detecting any increase in circulating levels of Y. ruckeri specific antibodies. Since both dendritic cells and M-like cells have been found in fish, is it suggested that gut-associated lymphoid tissue (GALT) associated with the gastrointestinal tract are involved in antigen uptake and generation of a local protective immune response against Y. ruckeri.