In order to evaluate the effect of josamycin (JM) therapy on the intestinal flora of yellowtail, Seriola quinqueradiata, bacteria were isolated from the intestines of 3 fishes before and after 3 days administration of 30mg/kg JM. Intestinal bacteria were cultured aerobically on BHI, BTB teepol, MacConkey and EF agar, and anaerobically on GAM agar. Sodium chloride was incorporated in all media at 2% except in BTB teepol agar at 3%. The effect of JM administration on the intestinal microflora was compared with that of ampicillin (ABPC) and oxolinic acid (OA). After 3 days administration JM, the number of viable bacteria on EF agar decreased but the number of viable bacteria on the other medium did not decrease. The identification of 50 isolates of intestinal flora was performed on the genus level. Vibrio was the predominant genus before and after JM and ABPC administration whereas, in OA administration, Streptococcus was predominant. Thus, oral JM administration was safe to yellowtail intestinal flora because only the bacteria belonging to the genus Streptococcus decreased.