The Cascade Range has generally partitioned sister species and populations into western and eastern forms, although north-south phylogeographic breaks within the Cascades have also been documented. We examined the relationships between and within 2 species of jumping mice, Zapus trinotatus (Pacific Jumping Mouse) and Z. princeps (Western Jumping Mouse), in Washington, Oregon, and southern British Columbia. From 24 localities we sampled 135 specimens and generated sequence data for 782 base pairs of mitochondrial DNA (mtDNA) control region and cytochrome-b. The 2 species were geographically partitioned to the east and west in concordance with the Cascades. However, in both northern Washington and southern Oregon, we found individuals with mtDNA haplotypes assigned to both species in syntopy, at the same sampling localities. Within Z. trinotatus, clades were further subdivided north and south, with a genealogical break across the Columbia River. Zapus princeps clades were also divided north and south, but with deeper divergences than within Z. trinotatus. We conclude that while the Cascade Range has contributed to the historic and contemporary partitioning of these 2 species, it does not represent an absolute barrier, as evidenced by syntopic localities.