Relative to other economically important marine animals, there is limited knowledge of the reproductive characteristics of the male Australian red claw crayfish Cherax quadricarinatus. In this study, the structure of the male reproductive system of C. quadricarinatus and the progression of spermatogenesis were examined histologically and using transmission electron microscopy (TEM). The male reproductive system of C. quadricarinatus is composed of paired testes, vasa deferentia, and genital appendixes. Each testis has a non-branched collecting duct and numerous attached seminal acini. Spermatogenesis occurs mainly in the seminal acini. Germ cell development is synchronous within each acinus, but not between different acini. Spermatids, formed after meiosis, undergo a complicated metamorphosis until the aflagellate spermatozoa are finally formed. Early in the metamorphosis, a highly convoluted membrane lamellar system is assembled in the cytoplasm as the chromatin begins to condense. The convoluted membrane lamellae split into numerous small vesicles which amass together and condense to construct a helmet-shaped acrosome cap, the concave side of which forms an acrosomal vesicle. In the meantime, the chromatin's decondensing expands the nucleus, while granular material, mitochondria, and membrane lamellae remain in the cytoplasm. Finally, the acrosomal cap and vesicle move to the surface and protrude through to the outside of the spermatid. The chromatin condenses again, enabling the nuclear volume to decrease and thereby forming a mature aflagellate spermatozoon.