The plant communities of Great Falls National Park, Fairfax County, Virginia, are described. Special attention is drawn to the habitat diversity represented, and the rocky slope-bedrock terrace community, which is found nowhere else in the county, is emphasized. An annotated list of vascular flora, which has been compiled from collections made from September, 1970 to May, 1972, is included. One hundred thirteen families with 640 taxa on the species or variety level are represented. Many species heretofore not reported from the county are reported.