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This thesis investigated the effects of selenium supplementation on the immune response in rainbow trout (Oncorhynchus mykiss). Therefore, as a start into selenium research, salmonid genomes were screened for selenoproteins with suggested roles in immunity that evaded previous automated annotation. Two salmonid-specific selenoprotein P genes were discovered, cloned, characterised and specific qPCR primers designed and validated. Next, cell culture experiments were performed to investigate the effects of Se supplementation on selenoprotein and immune gene expression in vitro. Several Se concentrations were tested and an optimal level for alteration of selenoprotein transcripts determined. This selenium supplementation level was then combined with immune stimulants to investigate the effects of selenoprotein expression in response to immune activation and immune gene expression following Se supplementation. Se supplementation exhibited a cell type-specific and context-dependent effect on the immune gene expression and was capable to induce or suppress the expression of several cytokines as a result of Se supplementation in vitro. The effects of supranutritional Se supplementation were then examined in vivo, following bacterial infection, injection vaccination and oral immunisation in three distinct in vivo trials. These experiments were designed to investigate the effects of selenium supplemented fish in comparison to unsupplemented fish in either unstimulated control groups (selenium effect) or immune activated groups (selenium*immunity). The Se supplementation was achieved by feeding a diet with Sel-Plex inclusion to 50 % of the fish in the experiment. Sel-Plex a dietary selenium supplement from Alltech Ltd and is comprised of mainly organic selenium derived from yeast cultured in Se enriched media. In the first trial, a bacterial infection with Aeromonas salmonicida was induced by intraperitoneal injection of the pathogen after 5 weeks of feeding the Se supplement. Then the innate immune response was examined by transcriptomic analysis of immune molecules and the selenoproteome. The results showed a significantly dampened cytokine induction in the head kidney of the Se supplemented group that indicated a reduced level of inflammation and suggested a swifter tuning of the immune response towards infection resolution prompted by dietary Se. This was accompanied by the induction of selenoprotein expression by infection in the same tissue, although not significantly different between diet groups. In the injection vaccination trial, an unadjuvanted experimental A. salmonicida bacterin vaccine was utilised that was specifically prepared for this purpose. The fish were vaccinated after 5 weeks of Se supplementation by injection of the vaccine. The immune response was examined by gene expression studies and revealed a more pronounced induction of cytokines in the liver of Se supplemented fish compared to the control group. Selenoproteins were equally induced for both groups in this tissue. Blood samples that were taken 4, 8 and 12 weeks post immunisation revealed significantly elevated serum levels of specific IgM in all vaccinated groups. The Se supplemented fish showed a significantly higher IgM titre in an analysis combining the three sample points. This finding could be significant during a disease challenge when a humoral antibody-directed immune response contributes to pathogen clearance. In the third trial, a primary immunisation with a commercial booster vaccine was attempted. This investigated the antigen uptake in the gut and any Se effects on this process. After 5 weeks of dietary Se supplementation the fish were orally immunised. Gene expression studies were performed on a subset of the fish that showed no marked differences between diet groups post immunisation. A distinct immune reaction was not induced by the vaccine. A subset of the fish was kept on the respective diets and challenged with a pathogenic strain of Yersinia ruckeri 53 days post completion of immunisation. No significant differences in survival between unvaccinated and vaccinated groups were found. Also, the diet groups showed no significant differences in level of protection. The orally administered primary vaccination did not protect from disease in a subsequent pathogen challenge which suggested an insufficient immune activation, as indicated by the gene expression studies. In this model, selenium supplementation did not enhance the antigen-induced immune response in vaccinated fish or level of disease protection in unvaccinated fish. Taken together, supra-nutritional Se supplementation of rainbow trout was shown to modulate the cytokine expression during a bacterial infection with A. salmonicida and enhanced specific serum IgM titres after injection vaccination with an A. salmonicida bacterin.