Mycorrhizal symbioses are generally very important in grassland ecosystems. The structure of arbuscular mycorrhizal fungal (AMF) communities varies with soil fertility in temperate grasslands, but relatively little is known about their ecology in tropical grasslands. We studied the biomass and species composition of AMF communities in grazed and un-grazed plots at eight grassland sites situated across natural precipitation and soil fertility gradients in the Serengeti, Tanzania. Biomass was assessed from direct measurements of AMF hyphae and spores in the soil. We also conducted a bioassay for densities of infective AMF propagules in the soil in which we measured mycorrhizal colonization in the roots of maize, a universal host plant, grown in soils from each of the 48 plots. Species composition was assessed from soil populations of AMF spores and also within buried mesh bags. We discovered that AMF species were individualistic in their responses to soil factors and grazing.  For example, spore abundance of Gigasporaceae was positively correlated with the availability of soil phosphorus and N:P ratio, and were greater in grazed versus un-grazed plots. This family of AMF is also very sensitive to soil N:P ratios and light availability in temperate grasslands. Our bioassay showed that mycorrhizae are very important for plant performance in Serengeti soil because maize biomass was significantly correlated with AMF hyphal density but not with soil nitrogen, phosphorus, or other abiotic variables. Thus, the structural variation that we observed in AMF communities is likely to influence the mutualistic function of AMF in the Serengeti.