In diverse mutualisms, it is common for potential partners to vary in the quality of benefits they provide. When weakly beneficial mutualists and parasites have a competitive advantage over strongly beneficial mutualists, it is not clear how strongly beneficial mutualists persist. If mutualism is destabilized by competitive superiority of weakly beneficial mutualists or cheaters, then mechanisms providing for stable coexistence among competing species may also provide for the persistence of mutualism. We analyze coexistence of species within a mutualist guild using a simple spatial model of patch occupancy to suggest hypotheses about the ecological persistence of mutualism in the interaction between plants and ectomycorrhizal fungi. We suggest that plants could facilitate the persistence of mutiralistic mycorrhizal fungi by enhancing the mortality of root tips colonized by competitively superior and less mutualistic fungi. Wc also discuss previous empirical studies and present original data from field observations in plant-ectomycorrhizal systems to address our predictions and to suggest profitable avenues for further work.