PREMISE: One of the main drivers of global change is biotic exchange, which leads to biological invasions. The genetic diversity and structure of invasive populations is influenced by multiple factors, most notably the details of a species' introduction, its pattern of range expansion, and its mating system. Taeniatherum caput-medusae subsp. asperum is a primarily self-pollinating, invasive, annual grass of the western United States (US). METHODS: Using historical information (herbarium specimens and published reports) and genetic (allozyme) data, we reconstructed the introduction history and pattern of range expansion of T. caput-medusae subsp. asperum in its invasive range. RESULTS: Herbarium collection data and published reports indicate that the grass was first collected near Roseburg, Oregon, in 1884 and then at Steptoe Butte, Washington, in 1901. Genetic analysis of 46 invasive populations of T. caput-medusae subsp. asperum detected seven homozygous multilocus genotypes (MLGs) across the western US. Several MLGs were found in localities associated with early collection sites. Only three of 1700 individuals we analyzed were heterozygous. Thus, high rates of self-pollination likely preserved the MLGs introduced into the western US and contributed to our ability to reconstruct the introduction history of this grass. CONCLUSIONS: Our data are consistent with the pattern associated with multiple introductions and local or regional range expansion. Despite multiple introductions, invasive populations of T. caput-medusae subsp. asperum exhibit relatively low within-population genetic diversity (i.e., invasive populations possess low evolutionary potential). Apparently, low within-population genetic diversity does not preclude the invasion of this grass in the western US.