The purpose of the present study was to determine whether dark and pale type A spermatogonia (Ad and Ap, respectively) are mitotically active during prepubertal development and whether proliferation of these germ cells during this protracted phase of primate development occurs predominantly during infancy before gonadotropin secretion is arrested. Four neonate (1–2 days of age), four infant (4–5 mo of age), and four juvenile (14–17 mo of age) rhesus monkeys (Macaca mulatta) were castrated 2 h after receiving an i.v. bolus of 5-bromo2'-deoxy-uridine (BrdU, 33 mg/kg body weight). Tissue was fixed in Bouin solution, and 5-μm paraffin sections were cut. Using periodic acid-Schiff reagent/Gill hematoxylin staining, the number per testis of Ad and Ap spermatogonia were determined. BrdU S-phase-labeled nuclei were identified using immunofluorescence. Conservative criteria were employed for classifying cell types, and this resulted in a fraction of A spermatogonia remaining unclassified. Ad, Ap, and the unclassified A spermatogonia each showed an approximately 4-fold increase over the 5-mo period from birth to infancy, and a similar increase was observed over the 10-mo period between infancy and the juvenile stage of development. Both Ad and Ap (and unclassified A spermatogonia) exhibited robust and similar S-phase labeling at the three stages of development. We conclude that the prepubertal expansion of Ad and Ap spermatogonia is achieved by mitotic proliferation that is relatively gonadotropin independent. This conclusion raises the question of the nature of the signal that arrests the cell cycle of Ad in adult testis.