Studies were conducted to determine the morphology and ultrastructure of spores of Glomus intraradices, a species of arbuscular mycorrhizal fungus that usually produces its spores inside the host root. Different wall structures are observed and wall development is described with both light microscopy and TEM. In mature spores the wall is formed by two zones: an outer, disintegrating zone and an inner zone with several layers. Mature spores have large centrally located lipid droplets and a peripheral layer of cytoplasm containing numerous nuclei, mitochondria, polyphosphate granules, and small lipid droplets.