Abstract: To determine the etiology of Pseudobagrus ussuriensis with skin ulcer, bacteriology was conducted from the edge of ulcer, the spleen and the kidney. Besides, wet mounts from gill, skin mucus and intestinal content were checked to exclude parasites. Pathological characteristics, target organs and visible pathogens were determined by histopathology. The results show that BHI is not a good medium for the first time bacteria isolation and culture from fish. No obvious bacteria were seen on the medium. The target organs were skin and muscle, spleen, head kidney and trunk kidney. All the fish mainly expressed necrotizing ulcerative dermatitis and panniculitis, splenitis and interstitial nephritis. Moreover, mild focal branchitis was also found. No obvious change was obversed in eye, brain, liver, heart, digestive tract and gas bladder. After re-staining the slides with Gimsa, we found long filamentous and hair-like bacteria in muscle tissue. Furthermore, fungal hyphae were also seen in muscle tissue. The etiology of this case was considered a co-infection of bacteria and fungi. The findings support the importance of histopathology in diagnosis.