Egg yolk immunoglobulins (IgY) were obtained from laying hens immunized with inactivated Aeromonas hydrophila. The purified IgY was shown to inhibit the growth of A. hydrophila in vitro and the optimum concentration for inhibition of A. hydrophila-specific IgY was 75 mg mL−1. In a subsequent challenge trial, 100 carp (200~250 g) were assigned to one of ten tanks with ten carp per tank. The fish in one tank were unchallenged whereas the remaining 90 fish were injected intraperitoneally with 100 μL of A. hydrophila at a concentration of 108 cfu mL−1. For the next 21 days, all fish were moved in their respective groups to a clean tank for 20 min day−1. The fish in four tanks (one unchallenged tank and three challenged tanks) received no treatment whereas the fish in the remaining six tanks were immersed in either 0.5 g L−1 aqueous nonspecific IgY (N = 3) or 0.5 g L−1 aqueous specific IgY (N = 3). Haemoglobin concentrations, white and red blood cell numbers as well as the mortality of specific IgY-treated fish were significantly different from those of the control. These results suggest that passive immunization by immersion with pathogen-specific IgY may provide a valuable treatment for A. hydrophila infection in carp.