SUMMARYCrystals at the surface of zygophores and sporangiophores ofMucor mucedowere investigated by electron microscopy, X-ray crystallography and chemical methods. Crystals on sporangiophores are spines, whereas those on zygophores have a tetragonal shape. Both types consist of calcium oxalate dihydrate (weddellite). Evidence is presented suggesting that the crystals are firmly anchored in, and in many cases are partly covered by, cell wall material.