Prairie Ridge State Natural Area, located in southeastern Illinois, is one of the most significant grassland habitat complexes in Illinois and well known for supporting the only population of Greater Prairie Chickens (Tympanuchus cupido) in the state. On the site, we found 544 vascular plant species, of which 438 are native (including the state-endangered Penstemon tubaeflorus), and one new state record (Schoenoplectus americanus). Exotic species accounted for 106 species (19.5%). Grassland communities were of two types: cool-season grasslands, managed by mowing and dominated by Agrostis gigantea, Bromus inermis, Dactylis glomerata, Festuca arundinacea, and Poa pratensis; and warm-season prairie grasslands, managed by fire or mowing, and dominated by Andropogon gerardii, Schizachyrium scoparium, and Sorghastrum nutans. Two small forest tracts were examined; both were immature second-growth mesic upland forests with an overstory dominated by Quercus alba, Carya ovata, and Acer saccharinum. Keywords: Grassland communities, Illinois, prairie chickens, prairie restoration