The control of a naturally-occurring outbreak of bacterial kidney disease (BKD)in masu salmon Oncorhynchus nasou was attempted by vitamin enrichment and by administration of ery-thromycin, using about twenty thousand hatchery-reared fish. Vitamin was ineffective in the control of BKD, since no difference in the mortalities between fish populations fed 40 days with pellet food supplemented with vitamins (A, D, E, B group, C, etc.) and fed with normal pellet food. The large alleviation in the mortality was recorded in about five thousand fish receiving an intra-peritoneal injection with erythromycin at a dose of 10mg per kg. Also, the incidence of BKD infection was markedly reduced in dissected fish samples. The reduction rates of the mortality and incidence were 1:4.5 and 1:4.6 in the duration of 20 days after the injection, respectively, compared with those of non-injected fish. Erythromycin conferred no influence (or damage) on the subsequent stages of eyed eggs and alevins from the injected fish. Thus, chemotherapy with erythromycin was considerably promising to control BKD infections in hatchery-reared masu salmon.