An introduced population of Etheostoma blennioides (greenside darter) in the Susquehanna River drainage has rapidly expanded its range since its discovery in the late 1960s. Geochronology of collection data in the Susquehanna River drainage provides evidence for explosive dispersal and long-range movement patterns. We discuss the current status and distribution of E. blennioides in the drainage, and suggest that other Mid-Atlantic Slope drainages with low native-darter diversity are at risk of similar colonization events. Although the long-term effect of these invasive populations is as yet unknown, we suggest that they may pose serious threats to persistence of several unique members of the Atlantic Slope ichthyofauna, especially the critically imperiled Etheostoma sellare (Maryland darter).