Bacterial cold-water disease (BCWD) causes heavy mortality of ayu Plecoglossus altivelis in Japan. To control BCWD of ayu, warmed water treatment at 28°C has been used. However, the mechanism of this treatment is unknown. Hence, we investigated the growth/survival of Flavobacterium psychrophilum at 15-28°C under two nutritional conditions: in modified cytophaga (MCY) broth and sterilized underground water. Within 2 days at 28°C in both the conditions, F. psychrophilum totally lost its colony-forming ability. We also studied distributions of F. psychrophilum in infected fish organs and in rearing water after various treatments. For bacteria detection, the colony-counting method and loop-mediated isothermal amplification (LAMP) method were used. For warmed water treatment, the rearing water was warmed from 18°C to 23°C or 28°C for 3 days. The treatments were started 1 day after immersion infection. In the experimentally infected fish, F. psychrophilum was detected neither in any fish organs nor in the rearing water after the 28°C treatment for 3 days. These results indicate the effectiveness of the warmed water treatment at 28°C against BCWD of ayu.