The present author has been performed isolation of Streptococcus sp. from the brain of diseased yellowtail (Seriola quinqueradiata) since January in 1979. This method has been useful to readily diagnose Streptococcus infection of fish with some pathological symptoms as exophthalmos, protrusive lesions on the inside surface of the opercle and the caudal peduncle, and epicarditis. Moreover, this method was confirmed to be applicable to infected fish which did not yet exhibit any pathological symptoms because of pure and sure isolation. This paper describes the results of the study on the bacterial isolation with the above mentioned method in 1979 and 1980.
