The genetic polymorphisms of the candidate markers of antimalarial drug resistance pfcrt, pfmdr1, pfatp6,and pfmrp1 were investigated in relationship with in vitro susceptibility of Plasmodium falciparum iso-lates and clinical response following artesunate (AS)–mefloquine (MQ) combination in 21 and 27 samplesobtained from patients with recrudescence and adequate clinical response, respectively. MQ (21.0 vs.49.9 nM) and AS (1.6 vs. 2.8 nM) IC50values (concentrations that inhibit parasite growth by 50%) weresignificantly higher in isolates collected from patients with recrudescence. Furthermore, a significantlyhigher MQ IC50was found in isolates from patients with recrudescence that carried pfmrp1 mutations atamino acid residues 191Y, 437A, and 876V. For AS sensitivity, a significant association was also detectedin isolates from patients with recrudescence that carried gene mutations at amino acid residues 437A and876V. MQ IC50of the isolates with recrudescence which carried ≥4 pfmdr1 gene copies was significantlyhigher than that carrying only one gene copy. In addition, a significantly higher proportion of isolatescarrying one gene copy was detected in the group with adequate clinical response compared with recru-descence. Results from this limited sample size suggested the potential link between pfmdr1 gene copynumber and pfmrp1 gene mutation, in vitro parasite susceptibility, and AS–MQ treatment response.© 2014 Elsevier B.V. All rights reserved.