Aims: To investigate the relationships between water temperature, bacterial growth, virulence and antigen expression in Vibrio salmonicida , the causal agent of cold water vibriosis in Atlantic salmon (Salmo salar L.). Methods and Results: The significance of sea temperature was investigated using historical clinical and oceanographic data. An upper threshold for disease of approx. 10degreesC was established. The effects of culture temperature and media type on bacterial growth were studied on solid and in liquid media. The highest rates of cell division were identified at 15degreesC on solid media and 10degreesC in liquid media. Outer membrane protein (OMP) expression and serological response in Atlantic salmon were studied using sodium dodecyl sulphate-polyacrylamide gel electrophoresis, Western blotting and enzyme-linked immunosorbent assay. A novel 76-kDa OMP produced in unshaken cultures at 10degreesC was not found to stimulate a specific humoral response. Conclusions: Diagnostic agar plate-based incubation of suspected V. salmonicida should be carried out at 15degreesC. High yield broth cultures for vaccine production should be incubated at 10degreesC or lower. Significance and Impact of the Study: This study is, to the best of our knowledge, the first to identify different optimal temperatures in a bacterial species cultured on physically different types of media. The evidence presented suggests that V. salmonicida and possibly other bacteria destined for vaccine use in poikilothermic organisms should be cultured at temperatures consistent with that at which disease occurs.