Abstract: More than 10 strains were isolated from the diseased parts of the postlarvae of shrimp Macrobrachium rosenbergii with opaque muscle disease.One of strains was proved to be the pathogen by challenge test.This isolate was classified as Staphylococcus xylosus by the testing of ATB Expression automatic bacteria identifying instrument. It was coccus)dia 0.8-1.1μm),Gram possitive,neither polar flagellum,nor capsule and spores.Acid production from glucose,fructose,maltobiose,lactose,trehalose,arabinose and mannite.ONPG test,superoxidase and ureaseand were possitive. Oxidase, coagulant of plasma, Voges proskauer reaction, arginine dihydrolase and ornithine decarboxylase were negative.Optimum growth temperature,salinity and pH range from 28℃-37℃,0%-2%,6-9,respectively.This isolated bacterium was highly sensitive to Drythromycin,Kanamycin Cegatexin and Gentamicin etc.