Technical Abstract: Understanding the relative importance of various functional groups in minimizing invasion by medusahead is central to increasing the resistance of native plant communities. The objective of this study was to determine the relative importance of key functional groups within an intact Wyoming big sagebrush/ bluebunch wheatgrass community type on minimizing medusahead invasion. Treatments consisted of removal of seven functional groups at each of two sites, one with shrubs and one without shrubs. Removal treatments included 1) everything, 2) shrubs, 3) perennial grasses, 4) tap-rooted forbs, 5) rhizomatous forbs, 6) annual forbs, and 7) mosses. A control where nothing was removed was also established. Plots were arranged in a randomized-complete-block with 4 replications (blocks) at each site. Functional groups were removed beginning in the spring of 2004 and maintained monthly throughout each growing season through 2009. Medusahead was seeded at a rate of 2000 seeds/m2 in fall 2005. Removing perennial grasses nearly doubled medusahead density and biomass compared to any other removal treatment. The second highest density and biomass of medusahead occurred from removing rhizomatous forbs (phlox). We found perennial grasses played a relatively more significant role than other species in minimizing invasion by medusahead. We suggest that the most effective basis for establishing medusahead-resistant plant communities is to establish 2 or 3 highly productive grasses that are complementary in niche, and that overlap that of the invading species.