Outbreaks of mycobacteriosis have been frequently observed in cultured yellowtail Seriola quinqueradiata in the northern part of Kagoshima prefecture, Japan. On the other hand, those outbreaks have seldom occurred inside of Kagoshima Bay. Antibody titers against the pathogen (Mycobacterium spp.) and frequency of detection of the pathogen from gill and kidney between those fish culturing ground showed no difference. However, water temperature was different in summer; 28°C in the northern part of Kagoshima pref. and 30°C inside of Kagoshima Bay. Then we injected the pathogen cultured at 25°C (optimum) or 30°C onto yellowtail maintained at 27-29°C and 17-23°C to confirm the influence of water temperature on outbreaks of mycobacteriosis. Cumulative mortalities showed 100% on both the 25°C- and 30°C- injected group maintained at 27-29°C, but the average days to death of the former group were significantly shorter than the latter. However, those maintained at 17-23°C were 30% and 0% for the 25°C- and 30°C- injected group respectively. These results show water temperature has an effect on the virulence of the pathogen and the disease resistance of yellowtail.