The seed of the Mexican species, Casimiroa edulis Llave et Lexarza is valued for medicinal purposes. We have studied its anatomy in different stages of development from ovule to mature seed. We applied general staining with safranin and fast green and specific staining for the detection of starch, lipids, lignin, polyphenols, and proteins. Embryo and seed development was also evaluated on the basis of volume. The ovule is sessile, bitegmic, crassinucellate, hemianatropous, with a nucellar curvature of 30-40º and a large hilum. The seed develops a large pachychalaza that covers approximately 70% of the surface. The testa is thin, nonlignified and tanniferous. The mature embryo occupies about 90% of the whole volume of the seed and contains lysigenous oil glands, starch grains and protein bodies. The radicle is not directed exactly at the micropyle. lt is located in the dorsal micropylar third of the seed. Casimiroa edulis had been described as lacking endosperm, but in reality the mature seed has a small amount of endosperm under the pachychalaza.