Abstract: A brown rot of straw mushroom(Volvariella volvacea)occurred commonly and caused serious damage in Guangdong province,Fujian province and Guangsi Autonomous Rigion.A pathogenic bacterium was isolated in this laboratory in 1985.Inoculation tests showed that the infected mycelium of the fungus stopped growing first and then rotted away,and the fruiting bodies turned yellowish brown to dark brown in color,then shrank in size and finally also rotted away.Based on the bacterium morphology,cultural charactristics,sresults of biochemical tests and serological test,the pathogenic bacterium was considered to be a Xanthomonas,sp,Further study on the identification of the species is in progress.