The donkey (Equus asinus) is a domesticated species from the Equidae family that is economically important and very well adapted to the arid regions of South America, Africa and Asia. However, except for a few studies in the literature related to testis structure and function in donkeys, to our knowledge there is no report regarding spermatogenic and Sertoli cell efficiencies in this equid species. Therefore, in the present study our main aims were to study these important parameters in Pêga donkeys. For this purpose, five sexually mature donkeys that had their testis perfused-fixed with buffered glutaraldehyde and routinely prepared for histological and stereological evaluations were investigated during the equine breeding period, which is from September to February in the South Hemisphere. The results found showed that, similar to most mammalian species already investigated, from the theoretical expected number, expressive germ cell loss occurred during the spermatogonial (~75%) and meiotic (~25%) phases of spermatogenesis, in such a way that, from each 10 spermatozoa expected to be formed from initial type A differentiated spermatogonia, only around 2 would be formed. However, mainly due to the higher Sertoli cell efficiency found for Pêga donkeys (~15 spermatids per Sertoli cell), spermatogenic efficiency (daily sperm production per testis gram) observed for this species (~42 million) is the highest observed for the domestic mammals already investigated, being ~110% higher than the value described in the literature for stallions.