Spermatogenesis is a productive and highly organized process that generates virtually unlimited numbers of sperm during adulthood. Continuous proliferation and differentiation of germ cells occur in a delicate balance with other testicular compartments, especially the supporting Sertoli cells. Many complex aspects of testis function in humans and large animals have remained elusive because of a lack of suitable in vitro or in vivo models. Germ cell transplantation has produced complete donor-derived spermatogenesis in rodents^,,,, but not in other mammalian species^,,. Production of sperm in grafted tissue from immature mammalian testes and across species has not yet been accomplished. Here we report the establishment of complete spermatogenesis by grafting testis tissue from newborn mice, pigs or goats into mouse hosts. This approach maintains structural integrity and provides the accessibility that is essential for studying and manipulating the function of testes and for preserving the male germ line. Our results indicate that this approach is applicable to diverse mammalian species.