The crystal structure of form III of syndiotactic poly(p-methylstyrene) is presented. Chains in nearly trans-planar conformation are packed in an orthorhombic unit cell with axes a = 13.36 Angstrom, b = 23.21 Angstrom, and c = 5.12 Angstrom. The crystalline density is 0.988 g/cm(3) with four chains in the unit cell (8 monomeric units); space group Pnam. Structure factors and packing energy calculations are shown. A refinement of the structure with the Rietveld method is also presented.