The spermatogonial populations in ten normal adult mice were analyzed using whole mounted seminiferous tubules. The undifferentiated A spermatogonia as well as the six generations of differentiating spermatogonia were clearly identifiable on whole mounts. Description plus quantitation of these cell types revealed that they behaved in essentially the same manner as their counterparts in the rat. Single undifferentiated A cells were classified as type As stem cell spermatogonia. They were distributed throughout the seminiferous epithelium, and by periodic mitoses, maintained their stock and furnished cells which would eventually differentiate. Although initially resembling the As spermatogonia, the progeny which were destined to differentiate were classified as type Aal spermatogonia because they were linked by cytoplasmic bridges, and because they usually underwent one or more synchronous mitotic divisions to form short chains of aligned cells. Ultimately, division of Aal cells were no longer seen, and the cells appeared to gradually acquire the typical morphological characteristics of A1 spermatogonia; these continued to differentiate according to the well-established pattern. It was concluded that the cyclic production of cohorts of A1 cells in this manner would ensure a continual supply of spermatogonia for differentiation.