The mycorrhizal fungus Glomus in association with bean hosts, Phaseolus vulgaris L., growing in pot cultures and grass hosts, Calamovilfa longiflora (Hook.) Scribn and Andropogon sp. growing on Lake Huron [Canada] sand dunes produced extensive external mycelium. This mycelium was the dominant factor in the aggregation of soil particles. Light and scanning electron microscope studies indicated that the sand grains were attached to the hyphae. An amorphous deposit often present at the interfaces of sand grains and hyphae apparently acted as an adhesive. Staining procedures indicated that this material contained polysaccharide. Other microorganisms were observed in association with the Glomus hyphae and the amorphous deposits.