Unidentified basidiomycete rhizomorphs growing on oak-leaf litter {Quercus alba) in Iowa and in Texas {Quercus gravesii) displayed arrays of crystals associated with their hyphae. X-ray diffraction and birefringence analyses identified the crystals as a mix- ture of calcium oxalate-monohydrate and -dihydrate. The Iowa oak-leaf-litter rhizomorph crystals occurred in two forms: young hyphae displayed either small styloid-like crystals oriented in all directions along the hyphae; or large clusters of elongated styloid-like crys? tals surrounding the hyphae, with individual crystals in each cluster displaying pyramidal ends. Crystals as? sociated with the Texas oak-leaf-litter rhizomorphs consistently covered all of the young hyphae and their tips with either small dagger-like crystals or thin, plate? like crystals whose margins were either smooth or fin- ger-like. Some larger crystal masses were also com- posed of crystals with pyramidal ends. The dagger-like and plate-like crystals were tentatively identified as the monohydrate form based on their higher birefrin? gence, whereas the crystals with pyramidal ends were identified as the dihydrate form based on their shape and lower birefringence. It is not known whether the two crystalline forms associated with the rhizomorphs are a function of the individual rhizomorphs, the litter source, the stage of crystal growth, or the ions present in the surrounding soil/ground water.