Well-defined lamellar crystals of polyglycine I (the beta modification of polyglycine) have been grown from a solution of a sample of 5000 molecular weight. The crystals are elongated and have an average thickness of 40 to 50 A. The unit cell is monoclinic and the crystals have a twined structure. Electron diffraction reveals that the polypeptide chains are normal to the basal plane of the crystals and that the hydrogen bonds are parallel to their long sides. From these data we conclude that crystals of polyglycine I have a folded beta structure.