Germ cell survival and development critically depend on the cells' contact with Sertoli cells in the vertebrate testis. Fish and amphibians are different from mammals in that they show a cystic type of spermatogenesis in which a single germ cell clone is enclosed by and accompanied through the different stages of spermatogenesis by an accompanying group of Sertoli cells. We show that in maturing and adult testes from African catfish and Nile tilapia, Sertoli cell proliferation occurs primarily during spermatogonial proliferation, allowing the cyst-forming Sertoli cells to provide the increasing space required by the growing germ cell clone. In this regard, coincident with a dramatic increase in cyst volume and number of germ cells per cyst, in Nile tilapia, the number of Sertoli cells per cyst was strikingly increased from primary spermatogonia to spermatocyte cysts. In both African catfish and Nile tilapia, Sertoli cell proliferation is strongly reduced when germ cells have proceeded into meiosis, and stops in postmeiotic cysts. We conclude that Sertoli cell proliferation is the primary factor responsible for the increase in testis size and sperm production observed in teleost fish. In mammals, Sertoli cell proliferation in the adult testis is not observed under natural conditions. However, on the level of the individual spermatogenic cyst—similar to mammals—Sertoli cell proliferation ceases when germ cells have entered meiosis and when tight junctions are established between Sertoli cells. This suggests that fish are valid vertebrate models for studying Sertoli cell physiology.