Swordtail fish (Xiphophorus helleri)were infected with 18 strains of fish bacteria .The result showed that the sensitivity of swordtail fish to the strains was coincident with that of the fishes from which the strains of bacteria were isolated or other sensitive fishes .It occurred notably stair-changes of dead rates to strong , medium strong and weak virulence strains .The appropriate route of swordtail fish infected with pathogenic bacteria was injection in the dorsal muscle .The individual difference of the high generation swordtail to the pathogenic bacteria was less than the primary generation .And it had shorter time from the first infected fish to death to the last one of the same group .The research suggests that the purebred swordtail fish have good prospects as an aquatic laboratory animal in detection of the virulence of fish pathogenic bacteria .