Leicht, S. A., J. A. Silander Jr., and K. Greenwood (University of Connecticut, Department of Ecology and Evolutionary Biology, 75 North Eagleville Rd. U-3043, Storrs, CT 06269). Assessing the competitive ability of Japanese stilt grass, Microstegium vimineum (Trin.) A. Camus. J. Torrey Bot. Soc. 132: 573–580. 2005— Microstegium vimineum (Japanese stilt grass) is an invasive grass in the eastern half of the United States which can form dense monocultures in forest understories, displacing native species. Although the loss of native species has been observed in the field, the actual competitive ability of this grass has not been examined. Microstegium vimineum was grown under controlled environment, greenhouse conditions in competition with Lolium perenne ssp. multiflorum (annual rye grass) and Muhlenbergia mexicana (Mexican muhly) in varying density ratios in full and low light treatments. Microstegium vimineum had a greater aboveground biomass, relative growth rate, and reproductive output than both competitors in both light treatments. The high competitive ability of Microstegium vimineum, especially in low light conditions, reflects its highly aggressive nature in forested or other landscapes of eastern North America.