The formation of calcium oxalate crystals in the developing bean seed coat was investigated by light, polarization, transmission, and scanning electron microscopy. Crystals first form in isolated lacunar parenchyma cells near the hilum, later in cells of subhilar tissue, and finally in hypodermal sclereid cells The initiation and development of crystals were studied in hypodermal sclereid cells Prior to crystal formation, young sclereid cells become highly vacuolate. Fibrillar material, as well as large vesicles apparently of cytoplasmic origin, appear in the vacuoles. Fusion or close association of these vesicles results in the formation of electron-dense bodies and membrane-like compartments (crystal chambers) within the vacuoles. Crystallization then commences within these chambers. Symmetrical prismatic twin crystals of calcium oxalate of two types form: kinked and straight twin crystals. The presence of an ordered substructure in crystal chamber sheaths indicates possible nucleation sites where crystallization occurs. Electron-dense bodies and fibrillar material within the vacuoles seem to be closely associated with crystal development. Mitochondria and plastids, the latter becoming transformed into crystalloplastids, also appear to be implicated in the crystallization process. Crystal growth is accompanied by thickening of cell walls which make contact with the crystals and begin to grow around them. After maximum crystal size and cell wall thickness have been attained, the remaining protoplast degenerates. Crystals in mature sclereid cells, surrounded by thick cell walls, could enhance the protective role of the bean seed coat.