In an earlier manuscript, we described the first phase of adaptation of the Palo Alto I strain of Plasmodium falciparum to Saimiri sciureus (squirrel monkeys). Now, after more than 50 P. falciparum blood transfers in splenectomized Saimiri, the parasite has become fully adapted to this experimental host. A highly reproducible pattern of infection is evident in these splenectomized animals, which is characterized by a rapidly rising parasitemia and a lethal outcome. In intact animals, the course of infection is extremely variable, with a tendency towards high parasitemias and low survival rates. After 60 passages, the parasites have maintained their invasiveness for human red blood cells and can easily be propagated by continuous in vitro culture. Conversely, prolonged in vitro culture of this parasite strain has not decreased its infectivity for Saimiri. Intact, P. falciparum-infected animals rapidly develop a high degree of resistance to reinfection, even when treatment is initiated shortly after detection of the first circulating parasites. Under identical conditions, splenectomized animals develop a variable degree of protection. The potential usefulness of squirrel monkeys as experimental hosts of P. falciparum and P. vivax malaria is discussed in light of the present findings.