The crystal structure of poly(2-cyano-1,4-phenylene terephthalamide) (CN-PPTA) has a monoclinic (metrically orthorhombic) unit cell with dimensions a = 9.21 angstrom, b = 5.08 angstrom, and c = 12.9 angstrom, containing monomer repeats of two chains. The space group is P2(1)/n, and the two chains in the unit cell are related by both the n-glide plane and the 2, screw axis. The calculated density is 1.447 g/mL, which is close to the observed density of 1.420 g/mL. A number of models were considered to accommodate the random 50/50 -CN substitution at 2- and 3-positions of the p-phenylenediamine segment. The data favor a structure in which 3-CN-substituted phenylenes are rotated by 180 degrees (about their 1,4-axes), so that effectively we have a mixture of 2- and 5-CN groups, which allows the substituents on adjacent chains to interleave. This structure would in fact be compatible with 2,5-disubstitution: random monosubstitution was modeled by 50% occupancy for the atoms of the -CN groups. LALS refinement led to a structure free of steric hindrance, with a crystallographic R value was 0.27. The phenylene-amide torsion angles are 50 degrees for the p-phenylenediamine segment and -30 degrees for the tereplithalic segment, values which are similar to those seen for Kevlar. The amide plane is rotated from the be plane of the unit cell by 11 degrees. The density is lower than that observed for Kevlar (1.50 g/mL) due mainly to the 50% vacancies at the CN sites. These vacancies probably facilitate penetration of the lattice by common polar organic molecules, leading to higher solubility.