The common marmoset (Callithrix jacchus), a small New World primate of high fecundity, is widely used in reproductive research. The aim of the present study was to determine the organization of the germ cells within the seminiferous epithelium, the duration of the spermatogenic cycle and the number of spermatogonial mitoses. Antibodies to cAMP response element modulator (CREM) and proliferating nuclear cell antigen (PCNA) and a cRNA directed against protamine P2 and morphological criteria were used to discriminate between stages of the spermatogenic cycle. Plastic sections were used to document the cell associations present in each of the nine stages of spermatogenesis. Up to five such stages could be observed within individual cross-sections of seminiferous tubules. Based on the pattern of incorporation of bromodeoxyuridine the length of the spermatogenic cycle was estimated to be 10 days and the duration of spermatogenesis to be 37 days. Four mitotic divisions were noted in spermatogonia. It is concluded that the organization of spermatogenesis in the marmoset has similarities to the human ('helical') and this makes the marmoset a suitable model for studies relevant to human testicular function.