Spermatogenesis in Rana esculenta is initiated during metamorphic climax and mature spermatozoa are present in froglets 45 days old. Cytological analysis of cell populations shows that some of the primary spermatogonia may lie dormant for brief intervals of time. Timing analysis of the process of spermatogenesis, in adults and in developing Frog larvae maintained at approximately 18°C, was investigated by different methods. The results are remarkably similar. Although perfect synchrony of the developing cells within a single germinal cyst is not the rule, a uniform rate of advancement of germinal cysts of the same stage of development, in most of the seminiferous tubules of a testis is evident. The duration of the secondary spermatogonial divisions is five to six days, involving five or six mitotic cycles, each cycle lasting approximately 24 h. The premeiotic S-phase, and phases of leptotene, zygotene, pachytene, diplotene+secondary spermatocytes, and spermiogenesis each have a duration, respectively, of six, two, six, twelve, two and seven days. The duration of spermatogenesis, from a “committed” primary spermatogonium to the formation of spermatozoa is 41 days.