The ultrastructure of the male gamete of Actinarctus doryphorus was investigated in the testis, seminal vesicles and seminal receptacles. In the testis and the seminal vesicles the male gamete is round-headed. Five spermatid stages are recognized. A large vesicle formed by the Golgi apparatus occupies up to 1/3 of the cytoplasm in spermatid stage 3. A rod-like acrosome originates in conjunction with the large nucleus and makes a characteristic bend of 180 degrees with respect to its origin. In the late spermatid development the nucleus and acrosome form a snout. The two mitochondria, which are located in the cytoplasma close to the flagellum, are not free, but form a "Nebenkern" resembling a Yin and Yang symbol. A midpiece is lacking and the flagellum is constituted of the traditional 9+2 microtubuli structure. In the proximal part of the flagellum 9 accessory osmiophilous fibers are pre sent. The spermiogenesis is very unique for a tardigrade, as the early spermatids are attached to giant nurse cells through cytoplasmatic bridges formed close to the spermatid nucleus. The nurse cell is characterized by a conspicuous labyrinth of rough endoplasmatic reticulum. In the posterior part of the testis spermatids are connected to each other in groups of 4+4. In the male, single spermatids/spermatozoa are found solely in the posterior part of the testis, seminal vesicles and sperm ducts. In the vesicles and ducts the gametes appear very mobile. Three stages of cuticular seminal receptacles were investigated in females. Newly moulted females lack spermatozoa in the receptacles. Females possessing one large oocyte with a chorion are characterized by having receptacles filled with spermatozoa. Moulting females, which have laid the egg have degenerated spermatozoa in the receptacles. The spermatozoon undergoes strong postcopulatory modifications. The large vesicle in the headregion disappears. The acrosome is oriented straight forward; the nucleus becomes rod-shaped, and the two mitochondria are difficult to locate. The spermatozoa in the receptacles are non-mobile and embedded in a strongly osmiophilous secretion. A full description of the two cuticular seminal receptacles is given, and the transfer mechanism of the male gametes to the female is discussed. The phylogenetic implications of this very advanced form of spermiogenesis and the complex transfer of gametes in a so-called primitive arthrotardigrade are discussed.