A strain of Plasmodium falciparum, designated Tanzanian I/CDC, from an American tourist returning from Tanzania, was isolated in vitro and in the Aotus monkey. Clinically, the infection showed a late recrudescent pattern of chloroquine resistance. In 2 inoculated Aotus monkeys, the infection recrudesced after a dose of chloroquine (40 mg/kg) curative for sensitive P. falciparum strains in the Aotus monkey. In 4 ,dditional monkeys two primary infections and one of the recrudescent parasitaemias were cured with a 100 mg/kg dose of chloroquine; the second recrudescent parasitaemia was cured with an additional 40 mg/kg dose of chloroquine. The 48 h in-vitro chloroquine-sensitivity test demonstrated that the Tanzanian I/CDC strain had a pattern of chloroquine resistance similar to a reference resistant strain, the Vietnam-Oak Knoll (FVO). These studies reinforce reports which suggest that chloroquine-resistant malaria is being transmitted in East Africa.