Exotic species potentially play an important role following wildfires. Their establishment has been documented widely in the past, though few research studies have data that span more than two or three years. This study was established following the Shirttail wildfire, which burned approximately 445 ha in Wind Cave National Park, SD in April 1991. In response to park managers’ concerns about the persistence of exotic species in the area burned by the wildfire, this project was designed to quantify the long-term population trend of two exotic plant species. In 1992, research plots were established in dense patches of common mullein (Verbascum thapsis) and Canada thistle (Cirsium arvense). Common mullein was by far the dominant species, in terms of cover, frequency, and density, in the mullein plots in both 1992 and 1993. However, by 1995, common mullein had nearly disappeared from the plots and Canada thistle had become the major species on these sites. By 2002, mullein cover and frequency averaged <1%, and thistle cover averaged 4%, but occurred in 33% of the sampling frames. On the thistle plots, density of both Canada thistle and common mullein increased from 1992 to 1993. In 1993, thistle densities averaged more than 12 plants/m² and mullein densities averaged >44 plants/m². By 1995, density of common mullein had declined to an average of about 8 plants/m², but Canada thistle densities remained in the 10 to 12 plants/m² range. In 2002, Canada thistle persisted at relatively low cover levels (<10%), but was present in around half of the quadrats and had densities averaging between 5 and 8 plants/m² on both seeded and unseeded plots. The persistence of these two species following wildfires is partly related to their differing life history strategies. Canada thistle spreads via extensive lateral root systems as well as by seeds, and its success is likely dependent on adequate soil moisture. Common mullein is an ephemeral species following wildfire, as it reproduces only by seeds. Long-term data such as these are crucial in developing management strategies following wildfires.