Although the goat is an economically important domestic mammal, there are very few data available in the literature for male reproductive biology in this species, mainly data related to testis morphometry obtained with high-quality light microscopy. Work recently developed in our laboratory found that the duration of the seminiferous epithelium cycle in goats is 10.6 days (França et al., 1999), and this finding allowed, for the first time, to estimate precisely the daily sperm production per gram of testis (spermatogenic efficiency) in this species and to compare this parameter with other mammalian species. Four sexuallymature Alpine bucks (Capra hircus; 25.3 ± 1.0 months of age and 59.3 ± 5.4kg of body weight) were utilized composing an homogeneous group.The mean testis weight in goats was 101g and the values found for seminiferous tubule and Leydig cell volume density (%) in goats were ~88% and ~1.5%, respectively. The mean seminiferous tubule diameter was 237µm, whereas the tubular length per testis and per testis gram was 1,829 meters and 20 meters, respectively. The number of round spermatids per pachytene primary spermatocytes (meiotic index) was 2.8, showing that, from the theoretical number of spermatids expected, 30% of cell loss occurred during the two meiotic divisions. The total number of germ cells and the number of round spermatids per Sertoli cell nucleolus (Sertoli cell efficiency) at stage 1 of the cycle were 24 and 15.4, respectively. This Sertoli cell support capacity is the highest among the domestic mammals species investigated. The Leydig cell volume was approximately 780µm3 and the nucleus volume 170µm3 . Both Leydig and Sertoli cells number per gram of testis were ~20 million, and the daily sperm production per gram of testis in goats (efficiency of spermatogenesis) was ~30 million. Several important parameters related to testis function such as the number of Sertoli cells per testis and sperm production, correlated with the volume occupied by blood vessels. In conclusion, this study shows for the first time that mainly due to high Sertoli cell efficiency and seminiferous tubule volume density, spermatogenic efficiency in goats is the highest among the domestic mammals already investigated.