The author studied the relation between abundance of year class of yellowfin tuna and surface water temperature in the tropical western Indian Ocean, and obtained following results:
1) The 1956 year class at both three and four years old were more abundant than the mean from 1955 to 1964 and the surface water temperature of the year in which the year class was born, was the lowest among those several years. The 1957 and 1958 year classes were less abundant than the mean and the surface water temperature in these years were higher than the mean among those several years.
2) There was no relation between the occurrence of the dominant year class and the degree of difference in surface water temperature between high and low temperature season in a year, and between low temperature season and high temperature season of next year.