The population genetic structure of Lutzomyia verrucarum (Townsend), a sand fly disease vector of Carrion's disease and cutaneous leishmaniasis in the Peruvian Andes, was characterized by sequencing 653 bp of cytochrome b and 1,125 bp of the NADH dehydrogenase subunit 4 genes of its mitochondrial genome. DNA sequence variation within and between valleys was compared in a sample of 220 sand flies from three valleys (Purisima, Huaylas, and Conchucos) and five departments (Amazonas, Cajamarca, Piura, Lima, and Huancavelica). Gene network and phylogenetic analyses indicated a high similarity of haplotypes collected within a single valley (0-0.52% nucleotide divergence). Flies from each valley had unique genotypes not shared with specimens from other valleys or from more distant regions (0.8-3.1% nucleotide divergence). Mountain ranges and geographic distance appear to have impeded migration (N(m) = < 0.18) between valleys and separated populations into discrete genetic units.