e report a natural infection by S. moniliformis in clean C57BL/6Jmice and the clinical. pathological,bacteriological as well as the epizoo-tical characteristicses were observed, which is the first time in China.Among the 14 of examined mice with clinical signs: 71.4% of tha miceshowed the swelling and purulence of the cervical lymph nodes,50.0% and21.4% denionstrated the swelling and purulence of the subcutaneous andhilar lymph nodes of lung, 21.4% showed the polyarthritis including hockand toe joints and 7.l% showed uterine empyema. The same bacterium wasisolated from the lesions of all infected mice and from the blood, spleen,kidney, liver and lung in some of the infected mice. The expsrimentalinfection with the bacterium isolated from the mice was successful and thetypical clinicopathologic signs such as septicemia, arthritis, congestion,edema and cyanosis were observed in SPF mice .The pathogenic characteri-stics of the bacterium was also the same as that of S. moniliformis.However the other 4 mouse strains which closely related to C57BL/6J suchas 615, BALB/c, CBA, KM mice, were not infected by the S.moniliformis.It demonstrated that the susceptibility to S.moniliformis is predominantlyinfluenced by differsnce of strains of the mice.