A disease characterized by white nodules in the kidney, swollen abdomen and ascitic fluid occurred in cultured ayu Plecoglossus altivelis from April to July, 2001. The bacterial isolate on KDM-2 from the affected ayu was identified to Renibacteirum salmoninarum using IFAT and PCR, and experimental infection resulted in the same clinical signs and re-isolation of the bacterium. From clinical signs, bacteriological examinations and challenge test, this disease in ayu was diagnosed as bacterial kidney disease (BKD). In experimental infection, cumulative mortalities of ayu and masu salmon Oncorhynchus masou were 100% and 60% when injected with 10⁷ cells/fish, and 100% and 0% when injected with 10³ cells/fish, respectively. These results indicate that ayu was more susceptible to the isolate than masu salmon, although there is no reported case of BDK in ayu. The affected ayu were originally introduced from another farm in April 2001, where salmonid fishes were reared in neighboring ponds, and it was revealed that masu salmon cultured in the latter farm was infected with R. salmoninarum. This result indicates that R. salmoninarum was horizontally transmitted from masu salmon to ayu.