Many grasslands have been transformed by exotic species with potentially novel ecological interactions. We hypothesized that exotic and native plant species differ, on average, in their percentage mycorrhizal colonization, and that mycorrhizal colonization is positively related to plant performance in the field. We compared colonization by arbuscular mycorrhizae (AM) fungi in perennial native and exotic species that were paired phylogenetically and by functional groups and grown under a common environment in field plots in Central Texas, USA. Roots were collected from plants in monoculture plots, stained, and percent colonization was assessed with a microscope. Aboveground biomass and dominance in mixture were used as measures of plant performance. Exotic species had significantly higher colonization of AM than native species, and this result was consistent across functional groups. Percent colonization was positively correlated with biomass and dominance in mixture across native species, but not across exotic species. Our results indicate that mycorrhizal dependence is a more important predictor of competitive balance among native than exotic plant species in the subhumid grasslands of the USA.