Natural antibody activity against Aeromonas salmonicida extracellular A-layer protein (A-protein) showed large individual variations in a farmed group of 101 goldfish (Carassius auratus L.). Statistical analyses of these variations led us to divide this group into homogeneous high and low naturally active (HNA and LNA) subgroups. The HNA fish were largely protected against experimental infection with a virulent atypical A. salmonicida strain, while 100% morbidity was recorded in the LNA group. In the course of active immunization with a particulate form of A-protein, a significant antibody response was exhibited by the LNA group only. Significance and implication of these results in vaccination practice are discussed.