An alternating copolyamide derived from 4-aminobutyric acid and or-isobutyl-L-glutamate 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. Additional data have been obtained from uniaxially oriented films by using X-ray diffraction. Experimental results agree with an orthorhombic unit cell of parameters a = 4.88, b = 14.0 and the chain axis c = 10.5 Angstrom. A sheet structure with intermolecular hydrogen bonds was deduced taking also into account infrared dichroism data. (C) 2000 Elsevier Science Ltd. All rights reserved.