The cycle of the seminiferous epithelium in M. o. ochrogaster was divided into eight readily recognizable stages based on the morphology of the developing spermatid. The mean relative frequencies of Stages I through VIII were 11.6, 19.25, 20.00, 19.00, 8.42, 6.17, 9.25 and 6.58%, respectively. The duration of one cycle of the seminiferous epithelium in this species, as determined from autoradiographs of thymidine-H3 injected testes was 7.17 days (S.E. +/- 0.03). This is the shortest spermatogenic cycle reported for any mammal to date. The approximate durations of the meiotic prophase, meiotic divisions, and spermiogenesis were 8.8, 0.4 and 12.2 days, respectively. The entire process of spermatogenesis was estimated to span approximately 28.68 days. The number of cycles that spermiogenesis spanned in this species was compared to values calculated for other species. Values within groups of related species appear relatively constant, but between groups the values are variable.