ABSTRACT It has been claimed that new germ cells are formed after maturity in certain female prosimians. A histological study was made of the ovaries of adults of four of these species (Galago crassicaudatus, G. demidoffi, Perodicticus potto and Loris tardigradus lydekkerianus), and of juvenile specimens of Loris. In addition, two adult female G. demidoffi were injected with 3H-thymidine and their ovaries subjected to autoradiographic examination. Mitotically active oogonia and/or oocytes in the early stages of meiotic prophase were present in all the adult prosimians examined; in G. demidoffi some of these cells incorporated 3H-thymidine, suggesting that DNA synthesis was in progress. These observations indicate that active oogenesis was taking place. There is no reason to believe, however, that new germ cells are formed other than by division of pre-existing oogonia.