In order to describe macroscopic and microscopic changes of testicles and propose maturation scale, Brycon siebenthalae male bred in captivity were collected and sacrificed monthly during two reproductive cycles. Testicles are structured organs with unrestrictive tubular operation. Spermatogenesis is cystic and develops in six phases: free primary spermatogonia and secondary spermatogonia, primary spermatocyte, secondary spermatocyte, spermatid and spermatozoid. Every seminiferous tubule contains multiple cysts and every cyst contains germ cells in the same development phase. Testicular maturation scale comprises four stages: immature or resting, in maturation, mature and regression. The greater gonadosomatic index coincides with the highest percentage of the mature stage.