Flavobacterium psychrophilum strains were isolated from ayu, pale chub and reddish bullhead caught in two rivers in Gunma Prefecture. The genotype of all the isolates was identified as A-S type being analyzed by PCR-RFLP and the banding pattern of genomic DNA after digestion with restriction enzyme BlnI on a pulse-field gel electrophoresis. Two isolates from pale chub and reddish bullhead were used for immersion infection to ayu to confirm pathogenecity, resulting 85% on higher mortalities in both strains. The results indicate that wild fishes infected with F. psychrophilum can be possible sources of the pathogen to infect to ayu.
