The crystal structure of the clathrate form of syndiotactic poly(m-methylstyrene) containing carbon disulfide is presented. The structure is characterized by polymer chains in s(2/1)2 helical conformation and CS2 molecules packed in an orthorhombic unit cell with axes a = 17.8 Angstrom, b = 13.1 Angstrom, and c = 7.8 Angstrom, according to the space group Pcaa. The calculated crystalline density is 1.14 g/cm(3) for two polymer chains (eight monomer units) and four carbon disulfide molecules included in the unit cell. As in the packing model proposed for the crystal structures of the beta class of the syndiotactic poly(p-methylstyrene) clathrates, the guest molecules occupy cavities delimited by isomorphous 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, instead, is similar to the close packing realized by the chains only in the clathrates and in the 6 form of syndiotactic polystyrene.