-Histopathology of experimentally infected ayu Plecoglossus altivelis with bacterial hemorrhagic ascites (BHA) was studied in order to determine its histopathological features. Fish (2.3 g in average weight, n=108) were infected with Pseudomonas plecoglossicida by immersion (IM) (ca. 10 7 CFU/mL), intraperitoneal injection (IPI) (ca. 10 4 and 10 7 CFU/fish) and intramuscular injection (IMI) (ca. 10 4 and 10 7 CFU/fish). The cumulative mortalities ranged from 92.9 to 100%.In the IM challenge groups, necrotic lesions accompanied by edema, fibrin deposition and hemorrhage were observed in the splenic and hematopoietic tissues. In some fish, the liver had necrotic lesions and microabscesses. In both the IPI and IMI challenge groups, lesions of the spleen and kidney were similar to those of the IM challenge groups. In addition, necrotic lesions and hemorrhage in the abdominal adipose tissue and pancreas were often observed in the IPI challenge groups. In the IMI challenge groups, the infected fish had definite lesions of the musculature at the injected site. The results revealed that the lesion formation in the spleen and kidney was the main pathological changes of BHA in ayu.