Castillo de San Marcos National Monument, St. Augustine, Florida, is administered by the National Park Service, United States Department of the Interior, and comprises 8.5 ha (20.5 acres) including the moat and expansive embankment (glacis) surrounding the historic fortress, as well as landscaping adjacent to parking lots and park headquarters. A floristic survey was conducted to provide park service personnel with a vouchered plant species checklist for the entire park, including the flourishing flora supported by the 10 m (33 ft) tall fort walls. Four intensive collecting trips conducted in 2003–04 yielded 153 species of plants in 129 genera of 61 families, including 56 species growing on the coquina walls of the fortress. Asteraceae, Poaceae, and Fabaceae had the largest number of species. Diagrams of the fort and photographs of representative plant species on the shellstone walls are also provided. Control measures for the wall vegetation are briefly discussed.