The spawning stock of North Pacific bluefin tuna (Thunnus thynnus) is caught by Taiwanese offshore longliners in the waters off eastern Taiwan every year between April and July. The catch of North Pacific bluefin tuna by Taiwanese longliners reached a historic high of 3089 metric tons in 1999, which exceeded that of the US, but was less than that of Japan. The fork lengths of our specimens ranged from 160 to 270 cm, and eviscerated weights were between 80 and 300 kg. The annual frequency distribution of eviscerated weight fluctuates somewhat. Scales from 228 fish were sampled for age determination, but only 133 which ranged from 204 to 255 cm fork length were readable. Under the assumption that one ring forms each year, the corresponding ages of the specimens were from 8 to 13 years. No significant linear relationship was found between fork length and scale radius. However, the greater number of rings which appeared in the scales, the bigger the fish was. The von Bertalanffy growth equation for the spawning stock of North Pacific bluefin tuna sexes combined was Lt = 366.681 × (1-e-0.086×(t+0.926)), as derived from the ageing results.