Aedes albopictus (Skuse), Asian tiger mosquito, a dengue fever vector, colonized the United States and Brazil ≈15 yr ago. The geographic origin and changes in population structure were examined using DNA sequences of mitochondrial NADH dehydrogenase subunit 5 (ND5). A 450 bp region was amplified by polymerase chain reaction (PCR) and screened for variation by both the single strand conformation polymorphism (SSCP) mutation detection technique and direct sequencing. The United States populations were typified by some local differentiation, but the most common and widespread haplotype was characteristic of the native range (Asian) populations as well. A comparison of the native range populations with the United States populations indicated a shared ancestral origin. The Brazil populations contained private (population specific) haplotypes, indicating no genetic exchange between the Brazil and the United States populations since introduction. Local differentiation was not observed among Brazil populations in contrast to the United States populations; this suggested distinctive dispersal mechanisms in the two countries. Extremely low variability was present in the A. albopictus mitochondrial sequences when contrasted with other taxa, reflecting a history of dispersal from a narrow genetic base. In contrast, relatively high levels of variation in several sequenced nuclear loci indicated the following colonization sequence: (1) the founding population was small, genetic drift had insufficient time to reduce variation at nuclear loci, and (2) populations successively expanded to new geographic areas established from a few founder females.