Susceptibility of three different stocks of ayu Plecoglossus altivelis (domesticated, amphidromous and landlocked stocks maintained in Hiroshima Prefectural Fisheries Experiment Station) to bacterial coldwater disease (BCWD) were compared by field spontaneous infection, laboratory water-borne infection and intraperitoneal injection with Flavobacterium psychrophilum. Cumulative mortalities of the amphidromous stock were significantly lower than those of the other two stocks in any infection experiments. The mechanisms underlying the resistance of the amphidromous stock against BCWD were investigated with respect to non-specific defense parameters such as respiratory burst, WST-1 reduction and phagocytic activities of the blood leukocytes and the serum-killing or static activity against the bacterium. However, no significant differences were observed in any parameters tested among the stocks of fish. It is suggested that the amphidromous stock is useful to establish a BCWD-resistant stock.