To assess the level of resistance to chloroquine (CQ) of Plasmodium falciparum in The Gambia in 1995-1996 we measured susceptibility in vivo by quantifying parasitaemia of children with mild malaria on days 4 and 8 after treatment. Pretreatment blood samples were used for susceptibility testingin vitro by the World Health Organization microtest and the prevalence of the tyrosine (tyry6 allele of the Pfmdrl gene was assessed by the polymerase chain reaction and restriction fragment length polymorphism analysis. Seven of 42 children (17%) treated with CQ remained parasitaemic on day 4 and required a change of antimalarial treatment. Susceptibility assays in vitro were performed on 50 I? falciparum isolates obtained from eligible children before treatment; 36 (72%) were resistant to CQ @ 1.6 prnol/L) . The median minimum inhibitory concentration (MIC) of artemether was 3.38 nmol/L (range 0.42-1351 nmoUL) and the median MIC of dihydroartemisinin was 0.88 nmol/L (range 0.22-14.04 nmol/L). Susceptibility in vitro to CQ and the pfmdrl genotype were determined for 31 fresh isolates.The allele was present in 12 of 22 isolates found to be resistant to CQ in vitro, but in none of the 9 isolates which were susceptible (Fisher's exact test, P=O.O05). All Rfalciparum isolates with the tyrs6 allele were CQ resistant in vitro, but since only half of all resistant isolates contained the allele, additional explanations for CQ resistance are required.