The crystalline form II of isotactic poly(vinylcyclopentane) was studied on the basis of fiber X-ray diffraction and of potential energy calculations of both isolated and packed chains. The hypothesis of a s(6*4/3) conformation of the chain has been investigated in detail. According to our interpretation, s(6*4/3) helices, which represent an anomalous case among isotactic poly(a-olefins), result from the superposition of a basically s(2*4/1) conformation of the backbone chain and of side groups following one another with a regular sequence of three conformations, mainly different for their orientation, Slight deformations of the main chain torsion angles from s(2*4/1) symmetry are found to be relevant to improve the data fitting. The resulting structure is indicative of a quite singular polymorphism, mainly ascribable to the different conformations accessible to the side groups.