A study was carried out to estimate the time for Nocardia seriolae infection of cultured yellowtail Seriola quinqueradiata. Yellowtail (58-1600 g in body weight) were collected at two fish farms in Ehime Prefecture from July 2001 to May 2002. Serum antibodies against N. seriolae antigens were detected by indirect enzyme-linked immunosorbent assay (ELISA). A significant increase in the serum antibody level was first recognized in August 2001, and high antibody levels were maintained in fish of both farms. Outbreaks of nocardiosis in the farms initiated in September or October and terminated in November. These results suggest that infection of N. seriolae in yellowtail began between July and August.