This work was undertaken to identify subpopulations of tunas by means of serological methods. Information concerning the blood types of albacore has been reported previously, (Suzuki, Shimizu and Morio, 1958; Suzuki, Morio and Mimoto, 1959). In these papers we reported two kinds of antigen, Tg1 and Tg2, and showed that albacore could be classified into four blood types, namely, Tg1 type, Tg2 type, Tg1.2 type and 0 type (lacking both antigens). Individuals having Tg1 antigen were found to be very rare in the Northwestern Pacific Ocean as compared with those from the Indian Ocean. The present paper reports on the continuation of tuna blood type studies and is concerned with individual variations in the erythrocyte antigens of yellowfin tuna, Neothunnus macropters (Temminck and Schlegel), and bigeye tuna, Parathunnus mebachi (Kishinouye). MATERIAL AND METHODS Blood samples of tuna were collected in the Indian and Pacific Oceans by fisheries research vessels. They were stored at - 20'C by glycerol-freezing (Cushing, Ridgway and Durall, 1957; Cushing, Fujino and Takahashi, 1959). Erythrocytes were recovered by dialysis against 1.5 per cent saline solution followed by several washings. Washed erythrocytes were used for immunization, agglutination and absorption tests. Normal sera of the marlins, kurokajiki, Eumakaira nigra (Nakamura), and shirokajiki, Marlina marlina (Jordan and Hill), were collected and centrifugally separated on shipboard in the Northwestern Pacific. The serum samples obtained were stored at - 20?C. Heteroimmune sera were prepared in rabbits by injecting about five ml of a ten per cent suspension of erythrocytes intravenously three times a week for a total of at least seven injections. Serum absorptions were performed at room temperature and at 0?C, following usual procedures. Agglutination tests were done at room temperatures by the holeglass method described in earlier papers.