Spermiogenesis and the stages of the seminiferous epithelium in the testis of the giant rat were studied in domesticated animals. 19 steps of spermiogenesis on the basis of the development of the acrosomic system and flagellum were observed, and the cycle of the seminiferous epithelium can be divided into 14 stages. Although the developing spermatids and spermatozoa of the giant rat differed structurally from those of the albino rat, the general trend of development and the cellular content of the seminiferous epithelium were similar in both species of animals.