Aeromonas salmonicida is a significant bacterial pathogen of cyprinid and salmonid fishes causing the systemic disease furunculosis. Several observations led us to believe that A. salmonicida was able to evade or suppress the immune system of the fish: injection of whole bacteria or surface antigens was unsuccessful at protecting fish against lethal challenges; memory did not develop in survivors of sublethal infections; diseased fish often carried other opportunistic bacterial pathogens in addition to A. salmonicida, and serum protein and particularly immunoglobulin significantly decreased during A. salmonicida infections. We tested the ability of fish sublethally infected with virulent and avirulent A. salmonicida to mount a humoral immune response to sheep erythrocytes and found fewer plaque forming cells in the pronephros and lower serum anti-SRBC antibodies in infected fish as compared to controls. We also monitored the cellular immune response of diseased fish by skin allograft rejection and found an enhancement of the response that increased as the disease progressed. However, the extend of inflammation was reduced in infected fish as compared to non-infected animals. At this moment these preliminary observations are difficult to explain. Our future research will focus more specifically on cell populations that may be affected by A. salmonicida.