The male genital system of Sousa chinensis was studied by histological and morphometric techniques.Sexual maturation was defined as the presence of spermatocyte sperm etc. in gonads.Evident difference was found in the histological structure of seminiferous tubules and epididymis between the immature males and mature males.Morphometric analysis showed that the diameter of seminiferous tubules of mature dolphins was 118.3±12.8 μm and the relative proportion of seminiferous tubules within the testis was 0.52.In immature males,the diameter of seminiferous tubules was between 47.4±3.5 μm and 60.3±6.0 μm,and the relative proportion of seminiferous tubules was between 0.27 and 0.40.