Marmosets are New World small primates phylogenetically close to humans and are commonly used in biomedical research. Although the reproductive biology of the common marmoset Callithrix jacchus is fairly well investigated, there are few data available for testis function for its close relative, Callithrix penicillata. In this regard, the present study was performed to investigate testis structure, spermatogenic cycle length, and spermatogenic and Sertoli cell efficiencies in eight captive C. penicillata. These animals received ³H-thymidine injections and had their testes perfused-fixed with glutaraldehyde and embedded in plastic at different time periods after ³H-thymidine injections, for histomorphometric and autoradiographic evaluation. The analysis of the different germ cell associations showed that two or more stages were observed in about 30% of the seminiferous tubule cross sections investigated. The values found for spermatogenic cycle length and for total duration of spermatogenesis in the marmoset C. penicillata, 15.4 and 69.3 days respectively, were very close to those cited in the literature for humans. However, the results observed for Sertoli cell efficiency (number of round spermatids per Sertoli cell; 8:1) and spermatogenic efficiency (daily sperm production per gram of testis; 18.4 million) were substantially higher than those observed for humans. The results found in the present investigation suggest that the black tufted-ear marmoset C. penicillata might represent an alternative and useful experimental model to perform comparative studies regarding the spermatogenic process, particularly in investigations related to the expansion of spermatogonial stem cells and the establishment of spermatogenic waves.