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Genetic analysis of the population structure of the western blacklegged tick, Ixodes pacificus Cooley & Kohls, was conducted using allozymes. This vector tick transmits the
Lyme disease spirochete, Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner, in the far-western United States. It ranges from British Columbia to Baja California and disjunct populations are present in Oregon, Nevada, Utah, and Arizona. Host-seeking adult ticks were collected from vegetation across the range of the species and were partially fed on rabbits prior to analysis. Twelve putative loci were resolved using starch gel electrophoresis. One locus, glucose-6-phosphate isomerase, formed an apparent north/south latitudinal cline and showed significant geographic structure. None of the remaining loci exhibited much genetic differentiation. Estimates of gene flow were high relative to other arthropods. Isolation-by-distance analysis suggests a recent and rapid range expansion. We conclude that the overall lack of differentiation is due high rates of gene flow.