Exoerythrocytic stage parasites of Plasmodium fieldi and Plasmodium simiovale, 2 simian malaria parasites related to the human malaria parasite Plasmodium ovale, were cultured in vitro by inoculating primary cultures of hepatocytes from rhesus monkeys (Macaca mulatta) with sporozoites. Less than 1% of sporozoites developed into schizonts for either species. Structure and size of the liver stages in both species were similar to previous in vivo descriptions, and the time required for in vitro maturation correlated well with the prepatent periods described for each species. Such monkey models could be very useful in conducting scientific investigations on the pre-erythrocytic stages of P. ovale-like malaria parasites.