The western black-legged tick, Ixodes paci®cus, is a primary vector of the spirochaete, Borrelia burgdorferi, that causes Lyme disease. We used variation in a 355-bp DNA portion of the mitochondrial cytochrome oxidase III gene to assess the population structure of the tick across its range from British Columbia to southern California and east to Utah. Ixodes paci®cus showed considerable haplotype diversity despite low nucleotide diversity. Maximum parsimony and isolationby-distance analyses revealed little genetic structure except between a geographically isolated Utah locality and all other localities. Loss of mtDNA polymorphism in Utah ticks is consistent with a postPleistocene founder event. The pattern of genetic di erentiation in the continuous part of the range of Ixodes paci®cus reinforces recent recognition of the di culties involved in using genetic frequency data to infer gene¯ow and migration.

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.

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