The direct fluorescent antibody technique was applied to a survey of “Pseudotuberculosis” in cultured yellowtail (Seriola quinqueradiata) in Miyazaki Prefecture. At the incipient stage of this infection, the characteristic lesions could not be found in spleens and/or kidneys of yellowtail macroscopically, although the causative bacteria could be detected in these specimens with the fluorescent antibody technique just as in conventional cultural procedure. It was established that the fluorescent antibody technique could be utilized as a diagnostic procedure of subclinical infection of “Pseudotuberculosis” in yellowtail.