We report the first detailed field survey of sickleweed (Falcaria vulgaris L.) in the United States. Sickleweed is native to Europe, the Mediterranean, Asia Minor, and Iran and was first reported in the United States in 1922. It is listed by the Nebraska Invasive Species Council as a Category II invasive plant species. In recent years, abundance and distribution of sickleweed has increased dramatically in and around the Fort Pierre National Grassland (FPNG), South Dakota. Management of such a rapidly expanding population is hampered by a general lack of baseline information on the biology and ecology of sickleweed. We used an environmental gradient approach to describe the abundance and distribution of sickleweed on the FPNG and found that sickleweed colonization and expansion may be largely driven by small and large scale disturbances that create gaps in ground cover (bare soil and total vegetative cover plus litter), reduced diversity (H'), and altered competitive relationships with western wheatgrass. Minimizing disturbances that create gaps in litter and vegetative cover, decrease diversity, increase bare soil, and alter the dominance of western wheatgrass may reduce the colonization and expansion of sickleweed.