The recent expansion of the nonnative invasive Asiatic sand sedge (Carex kobomugi Ohwi) at East Beach State Park, Rhode Island, is reducing populations of the most important native, dune-building species and their associated arbuscular mycorrhizal fungi (AMF). In contrast to the native American beachgrass (Ammophila breviligulata Fern.) that is dependent upon AMF to thrive in nutrient-poor sand dunes, C. kobomugi does not form beneficial associations with the fungi. Furthermore, assessments suggest that the sedge is competitively superior in obtaining the essential nutrient phosphorous without AMF-facilitation. Analysis of data from transects of the dune system revealed significant negative correlations between distributions of C. kobomugi and A. breviligulata that are being extirpated. Percent cover of A. breviligulata was significantly reduced in areas of C. kobomugi. Other native plant species were not significantly reduced as a result of C. kobomugi expansion. Spore populations of AMF showed significant positive correlations with percent cover of A. breviligulata and significant negative correlations with percent cover of C. kobomugi. Mean spore abundance of AMF in areas of C. kobomugi was less than in areas dominated by A. breviligulata. The number of species of AMF was not significantly reduced as a result of C. kobomugi likely because of highly aggregated and infrequent distribution of some species' spores. Assessment of mycorrhizal inoculum potential (MIP) of soils taken from the field mirrored the spore-population data: mean root colonization of plants grown in field soil of C. kobomugi (12%--24%) was between three and five times lower than that of plants grown in field soil of A. breviligulata (55%--72%). This study was unique in quantifying the effect of an invasive species on populations of mycorrhizal fungi in a dune habitat. It was novel in assessing the reduction of native plant and fungi species by C. kobomugi in Rhode Island. The replacement of AMF-forming species on dunes by a species that does not form AMF (and support spore production by these obligately biotrophic fungi) will have serious consequences when attempts are made to re-establish native species in the sites that are eventually cleared of C. kobomugi.