Young developing soybean seeds contain relatively large amounts of calcium oxalate (CaOx) monohydrate crystals. A test for Ca and CaOx indicated that Ca deposits and crystals initially occurred in the funiculus, where a single vascular bundle enters the seed. Crystals formed in the integuments until the embryo enlarged enough to crush the inner portion of the inner integument. Crystals then appeared in the developing cotyledon tissues and embryo axis. All crystals formed in cell vacuoles. Dense bodies and membrane complexes were evident in the funiculus. In the inner integument, cell vacuoles assumed the shape of the future crystals. This presumed predetermined crystal mould is reported here for the first time for soybean seeds. As crystals in each tissue near maturity, a wall forms around each crystal. This intracellular crystal wall becomes contiguous with the cell wall. Integument crystals remain visible until the enlarging embryo crushes the integuments; the crystals then disappear. A related study revealed that the highest percent of oxalate by dry mass was reached in the developing +16 d (post-fertilization) seeds, and then decreased during late seed maturation. At +60 d, CaOx formation and disappearance are an integral part of developing soybean seeds. Our results suggest that Ca deposits and crystals functionally serve as Ca storage for the rapidly enlarging embryos. The oxalate, derived from one or more possible metabolic pathways, could be involved in seed storage protein synthesis.