Crystalline hexamethylenediammonium adipate (m.p. 197 °C), the one to one salt of hexamethylenediamine and adipic acid, polymerizes at temperatures below its melting point yielding the corresponding polyamide (Nylon 66). Although there is considerable disorientation of the resulting crystalline polymer, two preferred directions of chain growth have been established both tilted at least four degrees from the unique b-axis of the monoclinic monomer. It has been found that contact with hydrochloric acid vapor catalyzes the polymerization of the monomer, resulting in appreciable polymerization at temperatures as low as 70°C and increased orientation of the polymer. Oriented reflections, attributable to the dimer and tetramer, are observed at all times in the acid catalyzed polymerization but not in the thermally polymerized material, indicating that continuous initiation is taking place in the former reaction.