Dispersal patterns during species' range expansions can strongly influence ecological traits of populations. Invasive species are ideal for studying the evolutionary ecology of species' range expansions. Because dispersal is difficult to measure directly, we used molecular markers as surrogates. We amplified microsatellite markers within and among populations of Brachypodium sylvaticum in Oregon, where it began expanding about 20 years ago. This grass species is aggressively expanding its range. We studied genetic diversity and dispersal in both central and peripheral populations at two geographic levels; 1) throughout Oregon, 2) along a small stretch of three parallel roads. Genetic diversity patterns are generally consistent with expectations; peripheral populations are generally genetically depauperate with high amounts of structure (avg. Fst=0.71, n=9), and central populations are more diverse with relatively low structure (avg. Fst=0.40, n=12). Patterns of dispersal were more complicated than we originally thought, with long-distance dispersal being common, at both geographic scales. Studies such as this one can help identify factors important to the ecology and evolution of successful invasive species, such as presence of multiple introductions and dispersal patterns. Research conducted early in the evolution of an invasive species may help predict other species that could become invasive, in addition to identifying populations that could be targeted for intense management.