The crystal structure of the beta class clathrate of syndiotactic poly(p-methylstyrene) containing benzene is presented and compared to that of the clathrate containing tetrahydrofuran, also belonging to the beta class. The structure is characterized by polymer chains, in s(2/1)2 helical conformation, and benzene molecules packed in an orthorhombic unit cell with axes alpha = 19.5 A, b = 13.3 Angstrom, and c = 7.7 Angstrom, according to the space group C222(1). The calculated crystalline density is 1.05 g/cm(3) for two polymer chains (eight monomer units) and four benzene molecules included in the unit cell. Our analysis affords an insight into the structural features typical of the beta class clathrates. As in the packing model proposed for the crystal structure of the s-PPMS clathrate containing tetrahydrofuran, the guest molecules occupy cavities delimited by isomorphic helices along the b axis and are related by the crystallographic 2-fold screw axis of the unit cell. The packing of the helices along the a axis is, instead, completely different in the two structures. bc layers of isochiral chains alternate with bc layers of chains having opposity chirality in the model proposed for the tetrahydrofuran clathrate. A new and unusual example of chiral crystalline phase, in which all the polymer helices assume the same chirality in the lattice, is proposed for the crystal structure of the benzene clathrate.