The effect of oral vaccines against bacterial fish diseases has been a topic for debate in many years. Recently both M-cells and dendritic cells have been found in fish and it is therefore likely that antigens can be taken up from the intestine and induce immunity in orally and anally vaccinated fish. The objective for this project is to investigate whether oral and anal vaccination of rainbow trout against Yersinia ruckeri O1 (biotype 1) causing Enteric Red Mouth disease (ERM) can protect rainbow trout against a subsequent experimental bath challenge.The rainbow trout were given oral vaccinations with AquaVacTM ERM Oral vet. (MSD animal health) or an experimental vaccine based on formalin killed Yersinia ruckeri O1, (biotype 1) bacteria. Eight groups were studied: 1) Control group (no vaccination, no infection), 2) infected control, 3) experimental vaccine, 4) experimental vaccine w/ booster (4 months post primary vaccination), 5) Experimental vaccine w/ 50 times higher dose, 6) AquaVac ERM (as a primary vaccine), 7) AquaVac w/ booster, 8) and one group which received a low bacteria dose of the experimental vaccine anally, as a primary and booster vaccination.The rainbow trout were bath challenged with 6.3 x108 CFU/ml Y. ruckeri 6 month post the primary oral vaccination. The challenge induced significant mortality in all infected groups except for the groups which received the experimental vaccine anally and in a fifty times higher concentration. These results show that a low dose of ERM bacterin fully protected rainbow trout when they are vaccinated anally. Oral vaccination can also induce full protection but the dose of the bacterin has to be 100 times higher than if the fish was to be vaccinated anally. This indicates that much of the oral feed bacteria is digested in the stomach of rainbow trout. This work has shown that it is possible to vaccinated orally against ERM, but the bacteria has to be coated in order to avoid digestion. Protection mechanisms will be discussed.