A infectious fungus (J131) recently isolated from artificial cultivation farms of Paecilomyces tenuipes is known to cause a great yield loss and deterioration of products. In this study, its occurrence and characterization were made. The isolate (J131) was identified as Paecilomyces farinosus (Holm ex S. F. Gray) Brown ＆ Smith. Colonies of the isolate on PDA grew moderately fast, attaining a diameter of 19mm within 14 days at 24${pm}$1$^{circ}C$. Vegetative hyphae are smooth-walled and hyaline. Conidiophores mainly arose from the submerged mycelium, usually measuring 1.3-1.5$mu extrm{m}$ wide and consisted of verticillate branches with whorls of 2 to 4 phialides. Phialides of 3.8-5.8${ imes}$1.5-1.8 $mu extrm{m}$, with a swollen basal portion tapered into a distinct neck of about 0.6-0.8 $mu extrm{m}$ wide. Conidia arisen from the phialides, were ellipsoidal to fusiform, smooth-walled, hyaline, 1.8-2.4${ imes}$1.1-1.4$mu extrm{m}$. .When silkworm larvae were injected with concentration of 1${ imes}$10$^$8/ conidia/$mell$ of infectious fungus J131, infection rate was 41.3％ and synnemata formation rate was 23.6％. Black spots were appeared on the surface of silkworm larvae in 5-9 days after injection. The surface of larvae was covered with hyphae which were gradually turned into yellow from white color during the incubation period of 3-4 days. 1 to several 4-6mm long synnemata of yellow or orange color were formed on the surface. The matured synnemata were ramified from its end and produced much conidia of white color.