Studies have been conducted on the temperature-dependent effect of bath vaccination of rainbow trout against Yersinia ruckeri O1. Protection of rainbow trout fry against challenge, following bath vaccination with a bacterin of Yersinia ruckeri O1, the bacterial pathogen causing enteric red mouth disease (ERM), was investigated at 5, 15 and 25° C. Rainbow trout fry were kept at controlled temperatures for two month before they were immersed in a commercial Yersinia ruckeri O1 bacterin for 10 minutes. Control groups were sham vaccinated using pure water. Fish were challenged with Yersinia ruckeri O1 one and two month post vaccination at the three temperatures. Protection of vaccinated fish was seen one and two month post vaccination in rainbow trout reared at 15° C. There was no effect of vaccination in rainbow trout reared at 5 and 25° C. Spleen tissue was sampled from 5 vaccinated and 5 control fish at each rearing temperature: 0, 8, 24 and 72 hours post vaccination in order to analyse immune gene transcript profiles with real-time RT-PCR (qRT-PCR). Complement factor C3 and C5 were down regulated in vaccinated fish kept at 5 and 15° C, relative to unhandled controls. Expression of the pro-inflammatory cytokines IL-1ß, INF-¿ and TNF-a, and the acute phase protein precerebellin, were up-regulated at all samplings post