A new poly(ester amide) derived from beta -alanine, 1,12-dodecanediol, and sebacoyl chloride has been synthesized and its crystalline structure studied using transmission electron microscopy and X-ray diffraction. The polymer crystallizes according to an orthorhombic unit cell with parameters a 4.80 Angstrom, b = 7.58 Angstrom, and c (chain axis) = 38 Angstrom. Simulation of the electron diffraction patterns indicates a molecular arrangement that combines features of polyamides and polyesters. Like the alpha and beta forms of nylons, intermolecular hydrogen bonds are established along a single direction that runs parallel to the a crystallographic axis (4.80 A). On the other hand, as the majority of polyesters, the primitive unit cell is constituted by two molecules with different setting rotation angles about c.