Type A spermatogonia in the mouse can be separated into five successive classes on the basis of nuclear morphology and stage of the cycle in which they occur. Enumeration of all types throughout the cycle of the seminiferous epithelium reveals that the A_s spermatogonia are the stem cells. They divide throughout the cycle and, especially at stages IX to I, form chains of cells which then give rise to spermatogonia A₁ at stages II–VIII. The A₁ cells divide in IX to form the A₂, which divide in XI to form A₃, and the A₃ cells divide in I to form the class A₄ spermatogonia. Spermatogonia A₄ give rise only to the In type; there is no evidence for the formation of either A_s or A₁ from A₄ spermatogonia. Repeated injections of ³H‐thymidine and tracing the history of labeled cells to 15 days after labeling supported the conclusion obtained from morphological and numerical data that A_s spermatogonia are the stem cells of the testis.