The differentiation of the male reproductive system, the copulatory structures, "red patch" and onset of sexual maturity were studied in Cherax quadricarinatus. For this characterization 353 males were observed, dissected and processed for histological analysis. Three developmental stages were identified. At stage I the testes are two parallel cords with a few lobes or acinii, connected medially by a short thin bridge (X-shape) and the vasa deferentia are straight transparent cords. In the second stage, more lobulated testes present an H-shape and the vas deferens begins to differentiate into three distinct regions: proximal, medial and distal. In the last stage testes are two parallel cords tending to fuse along their entire length and become a single structure. From this stage, the development of the "red patch" occurs and spermatophore formation is observed in the vas deferens giving it white colour. These primary and secondary sexual characters could indicate the onset of sexual maturity. The morphology of the copulatory structure is lobular at stage I and it differentiates into an elongated structure during stage II. This study shows that in C. quadricarinatus the differentiation of primary and secondary sexual characters of males starts simultaneously and very early in development (0.09-0.10g achieved nearly 1 month old) and that acquisition of sexual maturity is also an early event (6g achieved nearly 3 month old). The compared features within Astacidea and Decapoda and the possible applications in the analysis of phylogenetic relationships between different taxa are discussed.