(1) Age of bonito was determined on the basis of the vertebral bones just like that of chub-mackerel. The body length (l) of bonito well correlates with the length (T) of the cent-rum of the vertebral bone (Fig. 1). The rings (r) formed on the surface of the centrum can be considered as the year rings. The first ring measures 2•5mm in radius, the second one 3•9mm, the third one 5•4mm, and the fourth one 7•2mm. When the body length is 26cm, the length of the centrum (T) becomes equal to the radius of the first ring (r1) and thus the ring may be completed. Therefore, the bonito less than 26cm in body length may belong to 0-year group. According to the similar assumption, I-year group ranges in body length from 27cm to 34cm, II-year group from 35cm to 43cm, and III-year group from 44cm to 53cm. IV-year group may be larger than 54cm. (2) Most of the bonito caught by angling are mainly com-posed of III-and IV-year groups in the Liu-Kiu region, and III-year group occupies 60% of all and IV-year group 40%. While bonito shoal is simply composed of III-year group in the Tô-hoku region. It is also remembered that bonito caught by long lines is far larger in size than that by angling and usually belongs to V-year group or far older one. (3) There are two differ-ent shoals of bonito in these regions. The one is the migratory shoal and the other the resi-dent shoal. The resident shoal is gener ?? lly larger in the mean value and in the modal value of body length than the migratory group either in III-year group or in IV-year group. The migratory group is simply composed of III-year group in the Tôhoku region, while the resident group comprises to some extent IV-year group. The migratory group is higher than 20 in the quality-indicator (103. W/L3), while the resident group less than 20. In the Liu-Kiu region, the resident group is fished principally in the area around the small isles and over the shallow banks. The resident group is also related with small isles of the Bonin Chain, although it can migrate north-eastwards into the open sea of the Tôhoku region. On the other hand, the mi-gratotry groups are fished in any portion in both regions. (4) The migratory shoal of III-year proup in the Tôhoku region is probably originated from the migratory shoal of III-year group in the Liu-Kiu region, because shoal of similar character to those in both regions can be fished in the Seinan region. The migratory shoal of III-year group can be called the Liu-Kiu group both in Seinan and Tôhoku regions. While, the resident groups in both regions show no simi-larity in any respect to each other. In addition, no resident shoal is fished in the Seinan region between these two regions. According to the distribution of the fishing positions, the resident shoal seems to migrate into the Tôhoku regions from the southern sea through the Bonin Chain. Therefore the resident shoal can be considered as the Bonin group in the Tôhoku region. The migratory shoal occupies 60% of total catch and the resident shoal 40% in the Liu-Kiu region. In the Tôhoku region, the Liu-Kiu group occupies 80) of total catch and the Bonin group only 20%. (5) The fluctuation in the yield of these regions seems to be influenced by the changesin the age composition in the Liu-Kiu region, and also in the. ratio of the resident group (the Bonin group in the Tóhoku region) to the migratory group (the Liu-Kiu group in the Tôhoku region).