The 1-naphthylmethylammonium salts of not only (Z,Z)- but also (E,E)-muconic acid were found to polymerize upon photoirradiation in the crystalline state to provide a stereoregular polymer in a high yield. The stereochemical structure of both polymers obtained fron the monomers with a different configuration was identical, i.e., a meso-dusotactic-trans-2,5-structure. The stereochemistry of the monomer configuration and the molecular packing in the crystals.