Molecular tools to distinguish strains of Plasmodium vivax are important for studying the epidemiology of malaria transmission. Two sets of markers - tandem repeat (TR) polymorphisms and MSP3α - were used to study Plasmodium vivax in patients in the Peruvian Amazon region of Iquitos. Of 110 patients, 90 distinct haplotypes were distinguished using 9 TR markers. An MSP3α polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) using HhaI and AluI revealed 8 and 9 profiles, respectively, and 36 profiles when analyzed in combination. Combining TR and PCR-RFLP markers, 101 distinct molecular profiles were distinguished among these 110 patients. Nine TR markers arrayed along a 100 kB stretch of a P. vivax chromosome containing the gene for circumsporozoite protein showed non-linear linkage disequilibrium (I SA = 0.03, P = 0.001). These findings demonstrate the potential use of TR markers for molecular epidemiology studies.