The crystal structure of the clathrate form of syndiotactic poly(p-methylstyrene) containing tetrahydrofuran is presented. Polymer chains in the s(2/1)2 helical conformation and molecules of tetrahydrofuran are packed in the monoclinic unit cell with axes a = 18.8 Angstrom b =12.7 Angstrom, and c = 7.7 Angstrom, and with gamma = 100 degrees according to the space group P2(1)/a. Two polymer chains (eight monomer units) and four molecules of tetrahydrofuran are included in the unit cell. The calculated crystalline density is 1.13 g/cm(3). Tetrahydrofuran molecules occupy cavities delimited by the benzene rings of adjacent polymer chains along the b axis; each cavity is occupied by two molecules of tetrahydrofuran related by the 2-fold screw axis. A comparison with the crystal structures of the clathrate forms of syndiotactic polystyrene is presented.