Arbuscular mycorrhizal fungal (AMF) symbioses are important in grasslands.  Networks of AMF hyphae extend far beyond the rooting zone of their host plants and facilitate plant uptake of nutrients and water. The functional equilibrium hypothesis predicts that plants allocate relatively more to belowground structures (roots and associated mycorrhizal fungi) when they are more limited by belowground resources than by aboveground resources like sunlight. We tested this hypothesis by measuring the density of AMF hyphae in a long term grazing experiment in the Serengeti National Park, Tanzania. This experiment is composed of grazed and un-grazed plots at eight sites that are situated across natural precipitation and soil fertility gradients. We used a standard flotation and filtration method to measure soil densities of AMF hyphae, and also septate hyphae formed by saprotrophic fungi. Density of AMF hyphae was highest at sites with the lowest precipitation, soil nitrogen and plant biomass. Furthermore, AMF hyphal density was generally highest in the grazed plots. In contrast, density of septate hyphae was not significantly correlated with these factors. These results are congruent with the functional equilibrium hypothesis because AMF density was highest at the sites with the lowest soil resources and in the plots with the most sunlight. This pattern is expected if allocation to AMF biomass is tightly linked to resource limitation of host plants; while biomass of non-symbiotic saprotrophic fungi is less sensitive to the resource status of the living plant community.