Abstract: During our breeding of Bombyx mori, a pathogen was isolated from naturally dead bodies, all of which had a big bright coeruleus spot on their cephalothorax. This strain’s pathogenicity was confirmed by Koch’s law. In order to prevent and cure this disease of Bombyx mori, we’ve done some further studies and researches to the strain zl1-r. Physiological observation and molecular identification was performed to determine the taxonomic position of zl1-r. Antimicrobial Susceptibility Testing of zl1-r was sequentially carried out to find out an active method, which can control the disease. On the basis of phenotypic characterization and sequence similarity analysis, strain zl1-r was more closely related to the genus Aeromonas hydrophila. From the result of the Antimicrobial Susceptibility testing, we can conclude that the strain zl1-r is sensible to Gentamicin, Doxycycline, Streptomycin, Ciprofloxacin, Spectinomycin, Kanamycin, PolymyxinB and Ofloxacin, intermedio to Josamycin, Penicillin, Neomycin, Azithromycin and Roxthromycin, no sensible to Cefazolin and Sulfamethoxazole, resistent to Acetylspiramycin, Ampiocillin and Cefradine. Otherwise, we discovered that Ciprofloxacin and Ofloxacin had a very strong synergistic effect.