The in vitro activity of cycloguanil was assessed against 86 African isolates of Plasmodium falciparum by a semi-micro assay system. A bimodal distribution of susceptibility patterns was observed, with 44% of the isolates being cycloguanil susceptible. Cycloguanil alone retains a high activity against the intraerythrocytic forms of some isolates and, together with its activity against the hepatic stages, may be useful for chemoprophylaxis when combined with chloroquine.