The distribution of type A spermatogonia was studied using drawings of cross-sectioned tubules at various stages of the spermatogenic cycle of perfusion-fixed, epoxy-embedded mouse testis. Spermatogonia were classified as either positioned opposite the interstitium or opposite the region where two tubules make contact or in a defined, intermediate region at which the two tubules diverged. At stage V, the population of type A spermatogonia, comprised of A_s through A_al cells, is randomly positioned around the periphery of the seminiferous tubule. The A_s through A_al population becomes nonrandomly distributed beginning at stage VI, being located primarily in regions where the tubule opposes the interstitium, and remains nonrandom through stage III of the next cycle. The A₁ spermatogonia of stage VII, derived from most A_pr and A_al spermatogonia, and the A₂ spermatogonia of stage IX, derived from the A₁ spermatogonia, are also nonrandomly positioned opposing the interstitium. However, the A₃ population of stage XI becomes randomly distributed around the tubule. To our knowledge, these are the first data to show that the more primitive spermatogonial types (A_s to A_al) move to specific sites within the seminiferous tubule. Division of the regularly spaced, more primitive spermatogonia (A_s to A_al) leads to the spread of their progeny (A₁ to A₄) laterally along the base of the seminiferous tubule. The lateral spread from more or less evenly spaced foci ensures that spermatogenesis is conducted uniformly around the entire tubule. The data also suggest that the position of a seminiferous tubule in the mouse is stabilized in relationship to other seminiferous tubules.