The synthesis and a preliminary structural characterization of syndiotactic poly(m-methylstyrene) are reported. A complex polymorphic behavior has been revealed. Three different polymorphic forms and two clathrate structures, characterized by the inclusion of guest molecules in the crystalline lattice, have been found. The conditions for the obtaining of the various polymorphic forms, in unoriented and. oriented fiber samples, are described. Form I, obtained by crystallization from the melt or from the amorphous phase, is characterized by chains in the s(2/1)2 helical conformation. The same helical conformation has been found in the clathrate structures containing molecules of o-dichlorobenzene or benzene. Form II can be obtained by crystallization from solutions of solvents which do not form clathrate structure or by removing the guest molecules from the clathrate forms. Form III, obtained in oriented fibers by stretching amorphous films, is a mesomorphic modification characterized by chains in the trans-planar conformation with a high degree of disorder in the lateral packing of the chains. Annealing treatments or solvent-induced crystallization of the oriented mesomorphic form allow the obtaining of oriented fibers in some of the polymorphic forms. The differences and analogies with the polymorphic behavior of syndiotactic poystyrene and poy(p-methylstyrene) are outlined.