A preliminary model for the crystal structure of form III of isotactic poly(4-methylpentene-1) is suggested on the basis of structure factor calculations. Chains in 4(1) helical conformations are packed in a tetragonal unit cell with axes a = 19.38 angstrom and c = 6.98 angstrom; the space group is I4(1). The polymorphic behavior of poly(4-methylpentene-1) is discussed on the basis of conformational energy calculations on the isolated chain. The presence of different modifications is in accordance with energy minima present in the conformational energy map.