ABSTRACT:  Ectomycorrhizal fungi (EMF) form important mutualisms with plants, yet the factors that influence EMF diversity are poorly understood. Belowground interactions among plants and fungi may influence the structure of EMF communities, especially where the rhizospheres of foundation species overlap. We examined how neighbor identity affects EMF communities in habitat where pinyon pine (Pinus edulis) and ponderosa pine (Pinus ponderosa) are the only known EM-hosts. A co-dominant, one-seed juniper (Juniperus monosperma), and the herbaceous understory associate only with arbuscular mycorrhiza (AM). We compared the EMF communities of pinyon and ponderosa growing next to conspecific or heterospecific neighbors. We also quantified EMF species overlap when pinyon and ponderosa shared the same rhizosphere, and examined the influence of junipers on pinyon EMF communities. We found that 1) the diversity of EMF was not influenced by plant neighbor. 2) ponderosa EMF composition was influenced by pinyon neighbors, but pinyon EMF was not influenced by ponderosa neighbors - instead pinyon EMF communities were significantly influenced by nearby juniper. 3) when rhizospheres overlap, pinyon and ponderosa share 78% of their EMF species, suggesting a high potential for belowground hyphal connections. These findings support the hypothesis that neighbor identity affects EM community structure, and may help predict plant distribution changes during recent droughts that have caused high mortality of EM hosts.