The objectives were to investigate the changes in gonadal histology and plasma levels of gonadal steroids from the bisexual to the male phase in the juvenile male black porgy, Acanthopagrus schlegeli. The reproductive cycle in the adult females was also studied. The gonad consisted mainly of testicular tissue at 5 months of age. The ovarian tissue in the stage of primary oocytes was well developed at 7 months of age. The gonad underwent an intensive spermatogenesis at 9 months of age. During the first and second spawning season, the testicular tissue was dominant, which made the fish male. The ovarian tissue became dominant at 18 months of age during the non-spawning season. The chronological order of histological features in black porgy was suggested to be testicular, bisexual, male, bisexual, male, and female. Maximum plasma levels of estradiol-17β and testosterone occurred simultaneously before the first spawning season in juvenile males and might have an important function in spermatogenesis. For females, gonadosomatic index, oocyte diameter, and levels of estradiol-17β and testosterone increased during the spawning season. Peak levels of progesterone and 17α-hydroxyprogesterone were not observed in juvenile males and adult females during the spawning season.