A conformational analysis for poly(methylene-1,3-cyclopentane) (PMCP) for different microstructures is presented. Independently of the microstructure of PMCP, extended conformations suitable for the disordered crystalline phase are geometrically and energetically feasible. However, strong restrictions to the conformational freedom, both of the rings and of the backbone, are required. The similarity between the experimental diffraction profiles of a PMCP sample and the Fourier transform calculations for isolated PMCP extended chains confirms that the crystalline phase of PMCP consists of parallel extended chains (configurationally and conformationally disordered) with a pseudohexagonal arrangement of the chain axes but with a nearly complete intermolecular rotational and translational disorder along the chain axes. The large increases of the entropy of melting with all the examined kinds of configurational order (cis or trans or isotactic) would be due to the related higher conformational order in the disordered crystalline phase.