The structure of the male reproductive organs, and fine structural observations during spermatogenesis in Diplonychus esakii were described, including changes in nucleus, acrosome, microtubules and axoneme. Each testis comprised five follicles, which are a common characteristic of the genus Diplonychus in number and structure. The synaptonemal complex with a central element of many striations between two lateral elements was formed in early spermatocytes and two groups formed by a fusion of two synaptonemal complexes were still observed in spermatid. Mitochondria aggregated at one end of the late spermatocyte, and formed the nebenkern. The nebenkern divided into two mitochondrial derivatives and then elongated around the axial filament. The acrosome extended along one side of the nucleus and transformed into a hook-shaped throughout the spermatid. This type of the acrosome characterizes Hydrocoriomorpha. The acrosomal material was heterogeneous and very low in electron density. Numerous microtubules which appeared at the stage of nuclear elongation induced an elongation of the spermatid. The axoneme had the typical 9+9+2 microtubular pattern. Two structures were fairly clarified, the bridges from the mitochondrial derivatives to the axonemal microtubules nos. 1 and 5, and crystalline bodies within the mitochondrial derivatives. Aggregations of tubular RER were always associated with granular bodies in the spermatid. Their association may affect the formation of tubular RER from the granular bodies.