The crystal structure of form II of isotactic poly(4-methyl-1-pentene) is presented. Samples of isotactic poly(4-methyl-1-pentene) in the pure form II have been obtained by crystallization from tetramethyltin solutions. The crystal structure has been determined by using X-ray powder diffraction data and packing energy calculations. Chains in 41 helical conformation are packed in a monoclinic unit cell with axes a = 10.49 Angstrom, b = 18.89 Angstrom, c = 7.13 Angstrom, and gamma = 113.7degrees, according to the space group P2(1)/b. Four chains are included in the unit cell with a calculated density of 0.862 g/cm(3), according to the experimental density of 0.850 g/cm(3). Statistical disorder in the position of up and down chains in each site of the lattice is present in the structure. This structure provides an example of symmetry breaking in polymer crystals since the local 4(1) helical symmetry of the chains is not maintained in the lattice as crystallographic symmetry. As a consequence, the chains do not pack in a tetragonal lattice, as generally occurs for chains in 4(1) helical conformation but are packed in a monoclinic lattice having lower symmetry.