The crystal structure of the form I of syndiotactic 1,2-poly(4-methyl-1,3-pentadiene) has been determined studying the X-ray fiber diffraction pattern recorded onto a Fuji image plate. The chain conformation (T(6)G(2)T(2)G(2)), previously proposed by Meille et al. on the basis of the identity period, has been confirmed, but with deviations of chain torsion angles from the 60 and 180degrees values. The crystal system is triclinic, a = 17.513(8) Angstrom, b = 9.117(6) Angstrom, c = 11.251(14) Angstrom, alpha = 95.20(4)degrees, beta = 98.01(3)degrees, gamma = 92.74(5)degrees, and V = 1768(3) Angstrom(3). The space group is P (1) over bar, with two chains per cell, each formed by six nonequivalent CH2CH[CH=C(CH3)(2)] chemical units. The crystal structure was determined and refined by the least-squares method, keeping bond lengths fixed and assuming that chemically equivalent bond angles are equal to each other. The refined model shows unrealistic values for some side group bond angles, in particular for the angle between methyl groups. Quite satisfactory results are obtained fixing this angle to the value of 122.7degrees, as suggested by searching the Cambridge Structural Database. Final disagreement indices are R-1 = 14.3% and R-2 = 15.0%.