We have isolated 20 clones of Plasmodium falciparum from isolates from patients attending a village clinic in Sudan during 10 d in October-November 1989. The clones were genetically diverse, having highly variable molecular karyotypes and a wide range of drug responses. Chloroquine-sensitive (50% inhibitory concentration [IC50] in the 4-15 nM range) and chloroquine-resistant clones (IC50 in the 40-95 nM range) co-existed in the population, but no obvious amplification of the P-glycoprotein homologue gene, Pgh1 (previously known as the multi-drug resistance gene, mdr1) marked the chloroquine-resistant clones. Chloroquine resistance was reversible by verapamil in these clones, although they varied in their susceptibility to verapamil alone. These observations indicate that the biochemical characteristics of the Sudanese chloroquine-resistant P. falciparum are similar to those reported from south-east Asian and Latin American isolates, which is consistent with there being a similar molecular basis for this phenomenon.