A population-based field study was conducted in Zanzibar Town, Zanzibar, Tanzania, to assess the in-vivo and in-vitro susceptibility to chloroquine of Plasmodium falciparum. Single-dose therapy with chloroquine (10 mg base/kg) failed to clear parasitaemia in 11 of 22 (50%) treated subjects, and a standard therapeutic regimen of chloroquine (25 mg base/kg) failed to clear parasitaemia in 11 of 32 (34%) treated subjects. Concurrent in-vitro testing by the Rieckmann micromethod showed that 8 of 12 (66%) isolates were chloroquine-resistant.