The crystalline structure of form I of syndiotactic poly(para-methylstyrene) is described. In this form, the polymer chains assume a s(2/1) 2 helical conformation and are packed according to the space group P2(1)/a in a unit cell with a = 24.5 angstrom, b = 12.4 angstrom, c = 8.1 angstrom, and gamma = 143.5 degrees . The proposed model may be described by a stacking of layers of alternating right-and left-handed helical chains efficiently packed in an "interdigital structure" in which phenyl rings belonging to one chain interlock with the phenyl rings of the two adjacent chains of opposite chirality. A disorder in the correct succession of the layers is suggested. A possible similarity between this form and the nanoporous delta form of syndiotactic polystyrene, suggested by the fact they are both obtainable by guest removal from R class clathrates, is analyzed.