The alternating copolyamide poly(glycyl-12-aminododecanoic acid) (nylon 2/12),was prepared by polycondensation of the "dimer" glycyl-12-aminododecanoic acid pentachlorophenyl ester and subsequently studied by calorimetry, X-ray diffraction, and electron microscopy. Two crystal forms reminiscent of those characteristic of polypeptides and nylons, respectively, were found for nylon 2/12. Form I is made of fully extended chains arranged in hydrogen-bonded sheets which are staggered in a layered structure similar to the cu-form of nylons. Form II consists of a hexagonal array of 6-fold helices intermolecularly linked in a crystal lattice of a = 0.479 nm and c = 1.88 nm with features close to the peculiar polyglycine II crystal form. Evidence differentiating form II from the common gamma-form of nylon-12 was provided by a comparative electron; microscopy study carried out on single crystals of the two nylons in each respective form. : Nylon 2/12 melts at 232 degrees C and crystallizes, both from the melt and from solution, mainly inform II, which was found to be by far the predominant crystal structure adopted by this polymer.