We examined the prevalence of Flavobacterium psychrophilum infection in wild ayu Plecoglossus altivelis in four rivers in Miyagi Prefecture that have not been artificially stocked with juvenile ayu. The isolation rates of F. psychrophilum from apparently healthy ayu ranged from 58 to 100% (mean 89%). The majority of isolates were classified into the A/S genotype. All of the examined isolates were highly virulentto juvenile ayu. Our results suggested that bacterial cold water disease was present in wild ayu populations, even in rivers without a history of ayu stocking.