Samples of gonadal tissue and plasma of chub mackerel (Scomber japonicus) in Oita, Japan at the age of 2 were taken monthly. Gonadal tissue were fixed in Bouin's solution and then stored in ethanol 70 percent at room temperature for histological study. Plasma sample was kept at -sooe and analyzed by ELIZA The development of chub mackerel (Scomber japonicus) testes can be divided into 5 stages: Stage 1 (Immature), stage 2 (Early spermatogenesis), stage 3 (Late spermatogenesis), stage 4 (Mature), stage 5 (post spawning). Histological study showed that the spawning season of chub mackerel is from May to June when highest GSI value was found in May (19.4) and remained high up to June. Plasma 11-KT levels were significantly elevated in early spermatogenesis, and showed a peak (18.1 ng/ml) in late spermatogenesis stage and remained high during spermiation then dramatically declined (0.2 ng/ml) during post-spawning stage. On the other hand, 17.20 beta-P levels remarkably increased (0.4 ng/ml, P0.05) during spermiation, suggesting the involvement of 11-KT in spermatogenesis and the role of 17.20 beta-P in spermiation of male chub mackerel.