ABSTRACT-The sensitivity of yellowtail Seriola quinqueradiata and red sea bream Pagrus major to Nocardia seriolae was investigated. The LD 50 for yellowtail and red sea bream after being injected with N. seriolae was 1.1 ¥ 10 3 CFU/100 g B.W. and 3.4 ¥ 10 4 CFU/100 g B.W.,respectively. In an immersion challenge, the LD 50 for yellowtail was 1.2 ¥ 10 4 CFU/mL, while no mortality was observed in red sea bream at 1.0 ¥ 10 5 CFU/mL for 26 days. These data indicate that yellowtail is more sensitive to the bacterium than red sea bream. Serum bactericidal activity against N. seriolae was not observed in both fish species. Production of superoxide anion in kidney phagocytes isolated from red sea bream significantly increased in response to addition of live N. seriolae. However, such increase of superoxide anion production was not detected in yellowtail kidney phagocytes. This suggests that superoxide anion from phagocytes is one of important factors in protection against nocardiosis.