To determine whether genes flow freely between populations of the Ixodes ricinus-like ticks of eastern North America, and to determine whether the abundant northerly populations of these vectors of Lyme disease and other zoonotic infections may have arisen recently from a small cohort of ancestral founders, we characterized the nuclear IR27 microsatellite alleles in ticks sampled from the geographic extremes of their ranges. These microsatellite alleles differentiated populations located in the southeastern, northcentral and northeastern United States, respectively. Although evident heterozygous genotypes are about as frequent as would be expected in randomly mating populations, particular microsatellite alleles and diploid genotypes occur more frequently in certain populations than in others. Ticks from the Northeast and upper Midwest are markedly more related to each other than to ticks from the Southeast. Patterns of diversity present in this nuclear microsatellite marker correspond to those evident at a mitochondrial locus and indicate that the deer ticks of the Northeast and upper Midwest are genetically isolated from those in the Southeast. The Ixodes ricinus-like ticks that impose a public health burden in the northeastern and northcentral United States originated recently in a common founder population.