In this article a description of the process of spermatogenesis in the Chinese hamster is given. Spermiogenesis could be divided into 16 steps. The cycle of the seminferous epithelium was divided into 12 stages, coinciding with the first 12 steps of spermiogenesis. The relative and absolute duration of the stages was determined. The duration of the cycle of the seminiferous epithelium was found to be 17.0 days. The morphology of the spermatogonia was studied in seminiferous tubules mounted "in toto." Four classes of spermatogonia could be discerned; undifferentiated A spermatogonia (Ais, Apr, Aal), differentiated A spermatogonia (A1, A2, A3). In spermatogonia and B spermatogonia (B1, B2). It is interesting to note that the last generation of spermatogonia (B2) arises at the beginning of stage 7 and divides to give rise to primary spermatocytes in the second half of this stage; in most other species the last generation of spermatogonia arises in stage 4, giving rise to primary spermatocytes in stage 6. The unidifferentiated A spermatogonia were counted in six stages of the cycle, together with the differentiated A, In or B spermatogonia present in the same stages. The results of these cell counts are discussed in detail. One of the conclusions that could be drawn about the behaviour of the Ais and Apr spermatogonia during the cycle of the epithelium was that there is mitotic activity in these cells in several stages of the cycle. It is suggested that this mitotic activity serves to generate the Aal spermatogonia, which after one or more divisions transform into Al spermatogonia between stages 2 and 8.