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The percentage of fish attained sexual maturity was determined by histological examination of the gonads in red sea bream, Pagrus major, in various size classes of various ages. A total of 616 fish collected in March to June, 1985, was examined. A few fish mainly at the age of 2 years were found to have bisexual gonads, but their ovarian tissue was not functional. The sex ratio of fish of the ages of 2 to 6 years was assumed to be 1 : 1 through a binomial test. Male fish attained maturity ahead of females at the beginning of the spawning season, and the ratio of mature fish increased with advancing age rapidly in males and slowly in females. The proportions of fish having mature testes of the stage IV were 0.06 %, 79.0 %, 96.3 % and 100 % at the age of 2, 3, 4 and 5 years, respectively. On the other hand, the proportions of fish having mature ovaries of the stage III or IV were 28.6 %, 63.5 %, 72.6 %, and 95.0 % at the age of 2, 3, 4 and 5 years, respectively.