ABSTRACTAerial hyphae ofAgaricus bisporusgrown in agar culture, or on natural substrates like rye grain, produce abundant calcium oxalate crystals associated with the hyphal wall. These deposits, when first formed, consist of acicular crystals that cover the surface of the hyphae, giving the elements of the aerial mycelium a bottle-brush appearance. SEM examination of the crystal-bearing hyphae reveals that the crystals are arranged more or less tangentially on the hyphal surface. The crystals appear to originate within the wall of the hyphae, and as they increase in length their distal ends protrude through the hyphal wall. While the crystalline deposits of hyphae grown on malt-extract agar or on rye grain are typically elongate crystals, calcium oxalate deposits of hyphae grown on enriched media consist of both elongate forms and large, plate-like crystals. Scanning and transmission electron micrographs, energy-dispersive x-ray elemental analysis, and x-ray powder diffraction analysis of these crystals are presented.