This paper reports the resistance variation of wild, mirror and jian common carps to the outbreak-fish-disease caused by the Aeromonas hydrophilla. When the bacteria concentration was 6. 0 ×10⁸ CFU/ml (four gradients at 10⁰, 10^-1, 10^-2 and 10^-3), injection dosage was 0. 3 ml/fish, the median lethal dose (LD₅₀) for wild carp, mirror carp and jian carp was 10^{-0. 976}, 10^{-0. 562} and 10^{-1. 375} respectively, which indicated that there was a significant difference in the median lethal dose among strains (F>F_0.05). The mechanism of resistance was studied from 4 sides: In sides of leucocyte phagocytic function and bactericidal reaction by alternative pathway of complement (C₃ shunt), mirror carp was higher than wild carp and jian carp without significant differences (F0.05); in the sides of erythrocyte C_3b receptors-aggregates and the total amount of complements, the rank was jian carp>wild carp>mirror carp at a high significant differences (F>F_0.01 ). Erythrocyte C_3b receptorsaggregates and the total amount of complements are two important indicators which reflect the immune capacity of fishes. This study found that the jian carp is little higher in these two indicators, but it is more infectious to the pathogen of outbreak-fish-disease, Aeromonas hydrophilla, this situation represents that different strains of common carp have a within species heterogeneity to outbreak-fish-disease.