A study of resistance of three strains of crucian carp (Pengzhecrucian carp, allogynogenetic crucian carp and white crucian carp) to the outbreak-fish-disease was conducted. When the concentration of the diseases eaused bacte-ria-Aeromonas hydrophila was 6.0×10~8 ind./ml, injection dosage was 0.3 ml/fish,the median lethal dose (LD_(50)) for Pengzhe fish was 10~(-1.54), allogynogenetic fishwas 10~(-1.61), white fish was 10~(2.21), the differences were high significant (P0.01). The mechanism of resistance was studied from three sides: (1) The totalamount of camplements-the observed value in Pengzhe fish was 6.40 unit/ml,allo-gynogenetic fish was 5.97 unit/ml, white fish was 2.20 unit/ml, the differencewas highly significant. (2) Bactericidal reaction by the alternative pathway ofcomplement(C_3 shunt)-- Pengzhe and allogynogenetic were better than white highlysignificant (multiple mean comparison, DD_(0.01)). Pengzhe fish better than allo-gynogenetic fish, but not significant. (3) Leukocyte phagocytic function-therewere a high significand difference among the 3 crucian carps (P0 .01). In thephagocytic percentage, Pengzhe fish better than white and allogynogenetic fishhighly significant (DD_(0.01)), in the phagocytic index,the Pengzhe and allogyno-genetic fish better than white significantly (DD_(0.05)). Pengzhe fish is the betterstrain with high resistance to the outbreak-fish-disease, white fish is the worst.