ABSTRACT: Brook charr Salvelinus fontinalis, both those susceptible and those resistant to the pathogenic haemoflagellate Cryptobia salmositica,responded well to a commercially available Aeromonas salmonicida vaccine. The proportion of susceptible and resistant charr which produced agglutinating antibody were similar, but resistant brook charr had higher agglutination titres than susceptible brook charr. Concanavalin A stimulated peripheral blood leucocytes of Cryptobia-resistant charr and the kinetic profiles suggest that maximal stimulation is dependent on mitogen concentration. Cryptobia-resistant brook charr displayed both cell-mediated and humoral responses suggesting that their immune functions are similar to those of Cryptobia-susceptible charr.