Abstract: The male reproductive system of Daphnia (Ctendaphnia) carinata is composed of a pair of testes, deferens and one male gonopore. The wall of the sausage-shaped testis is composed of connective tissue and germinal epithelium inside. The tube-shaped deferens, which gets larger at the end of it, is mainly made up of connective tissue. The spermatogenesis includes five stages: spermatogonium、primary spermatocyte、secondary spermaticyte、spermatid and spermatozoon. The mature sperm is clubbed and the spermatogenesis shows a tendency of synchronization. The sexual maturation experiences four stages. The male reproductive system retains its primative characters on structure and function, but it has also made up its adaptability through the long-time evolution, which enables Daphnia (Ctendaphnia) carinata and other species of Cladocera to live with the changeable living environment.