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High variability in plant-growth response to the presence of different mycorrhizal fungi can be a major determinant of local plant species diversity. Multiple species of arbuscular mycorrhizal fungi can coexist in terrestrial ecosystems, and co-occurring plants can differ in their response to colonization by these different fungi. However, the range of mycorrhizal plant-growth responses that can occur within communities has not been determined. In the present study, I crossed a large number of plant and fungal species that co-occur to determine the range of responses that can exist within an ecosystem. I also crossed exotic fungal isolates vs. local plant isolates and local fungal isolates vs. exotic plant isolates to determine whether the range of plant growth responses differs when using foreign genotypes. The data indicate that plant growth responses to mycorrhizal inoculation within an ecosystem can range from highly parasitic to highly mutualistic. In this study, the direction and magnitude of the response depended on the combination of plant and fungal species. No plant did best with the same fungal isolate. The range of responses was greatest when using local plants and fungi. Whereas parasitic and mutualistic responses were also detected when using foreign plant or fungal genotypes, the range of responses was significantly reduced, as was the relative frequency of positive responses. Overall, this study suggests that, within ecosystems, arbuscular mycorrhizal fungi can function along a continuum from parasitism to mutualism, and that extreme responses are more common when using locally adapted plants and fungi. This high variation in plant growth response may be a large contributor to plant species coexistence and the structure of plant communities.