Etheostoma caeruleum (Rainbow Darter) was first collected from the Potomac River drainage in 1977 and has subsequently expanded its range to include nearly all major tributaries in the system. Analysis of the chronology of Rainbow Darter collection data demonstrates the explosive nature of this range expansion. Using this chronology, we assert that Rainbow Darter is not native to the drainage and likely occurs as a result of human-induced interbasin transfer. Although the ecological impact of non-native Rainbow Darters in the Potomac is unknown, the potential exists for competition with native species. The presence of numerous instream blockages prohibiting colonization of portions of the drainage make this system well suited to study the impacts of introduced darters on native ichthyofauna.