Ecologists have long been interested in factors that control the structure of communities and the relative importance of top-down effects of predators versus bottom-up effects of resources. There is a growing body of evidence that microbial symbioses are important determinants of plant community structure and indirectly affect herbivore and predator trophic levels. Studies with mycorrhizal fungi, N-fixing bacteria and endophytes of grasses have demonstrated that they can affect competition, coexistence, soil nutrient dynamics and plant-herbivore interactions. Long-term field experiments with one grass/endophyte interaction suggest that grassland community structure is determined by the fungus. While total plant productivity of experimental plots was similar, the composition of the vegetation was altered by endophyte symbiosis. The host grass tall fescue (Festuca arundinacea) dominated plots when infected while other grasses greatly increased in uninfected plots. Indirect evidence suggests that changes in prairie vole (Microtus ochrogaster) grazing patterns and reproductive physiology may be in part responsible for vegetational changes. These results provide evidence that, in addition to bottom-up and top-down forces, microbial symbionts of plants are important determinants of community structure.