The alternating copolyamide, poly(L-alanyl-11-aminoundecanoic acid), has been synthesized by the active ester method, and subsequently characterized. The structure and morphology of lamellar crystals have been investigated by using transmission electron microscopy, selected-area electron diffraction and X-ray diffraction. Additional data have been obtained from uniaxially oriented fibres. The deduced unit cell parameters (a = 4.79, b = 10.35, c = 29.8 Angstrom, and alpha = beta = gamma = 90 degrees) and the C222(1) space group symmetry indicate a layered structure which is related to that found in poly(L-alanine). A folded conformation is also deduced for the polymethylene segment. Results obtained from enzymatic degradation are also reported.