Description of the female of Quentius kozeki n. g., n. sp., parasitic in Marmose at Cali (Columbia). Because of its cephalic structure, the species is considered to be the most primitive Rictulariidae actually known. In addition, however, it has aberrant and specialized characters. The existence of this species seems to indicate that the rictularioids were established as early as the Cretaceous in American marsupials. They could have been maintained in these animals at least until the Oligocene before invading Northern rodents of the Nearctic region. From these hosts, as Quentin (1971) demonstrated, there was a North-South evolution throughtout the world.