The immune response in rainbow trout against a bacterin of Yersinia ruckeri, a bacterial pathogen causing enteric red mouth disease (ERM), was investigated at 5, 15 and 25° C. Rainbow trout were immunized by i.p. injection of a Y. ruckeri (serotype O1) water based bacterin and compared to control groups injected with phosphate buffered saline (PBS). Blood and tissue samples (spleen and head-kidney) were taken for subsequent analysis using solid phase enzyme-linked immunosorbent assay (ELISA) and real-time PCR (RQ-PCR), respectively. The up-regulation of cytokine genes was generally faster and higher at high water temperature with major expression at 25° C. The pro-inflammatory cytokine IL-1ß and INF¿ was significantly up-regulated in all immunized groups whereas the cytokine IL-10 was merely up-regulated in fish kept at 15 and 25° C. The gene encoding the C5a (anaphylatoxin) receptor was expressed at a significantly increased level in both head-kidney and spleen of immunized fish. The secreted IgM encoding gene was significantly up-regulated in the head-kidney of immunized trout reared at 25° C, and a positive correlation (r: 0.663) was found between gene expression of secreted IgM in the head-kidney and Y. ruckeri specific antibodies in plasma measured by ELISA. However, no regulation of the teleost specific immunoglobulin IgT, which was generally expressed at a much lower level than IgM, could be detected. The study indicated that both innate and specific adaptive immune response factors are highly temperature-dependent in rainbow trout.