Little is known about the factors influencing the species composition of arbuscular mycorrhizal fungal (AMF) communities and the functional significance of community differences.  We compared the function of AMF communities associated with the same two grasses in an East Coast serpentine grassland (Nottingham, PA) and in the tallgrass prairie (Hayden, IA).  The AMF morphospecies present at the two sites are similar except that Hayden is richer in Glomus spp. and lacks Gigaspora gigantea, which is common at Nottingham.  In a greenhouse experiment, we compared host plant shoot biomass when plants and soils were matched with their native AM fungal community versus when they were matched with the AMF community from the other site.  Sorghastrum nutans produced greater shoot biomass with its native AMF community.  This was true both for prairie plants growing in prairie soils and for serpentine plants in serpentine soils.  The same trend was apparent for Schizachyrium scoparium, but it was not significant.  Thus, the AMF communities appear to be ecologically matched to their plants and soils.  These functional differences could be due to species composition differences between the AMF communities or different ecotypes of the same AMF species.  The plants and soils also influenced the species composition of the AMF community.  Starting with the same AMF community as inoculum, community composition diverged with foreign plants and soils compared to native plants and soils.  This study provides evidence for environmental controls on AMF community composition and its importance for host plant performance.