The crystal structure of an alternating isotactic ethylene (E)-cyclopentene (C) copolymer (ECC) has been studied on the basis of X-ray diffraction, geometrical, and conformational analyses of the single chain and packing energy calculations. A value of chain axis periodicity of 9.0 A has been evaluated from X-ray diffraction patterns. Geometrical and conformational energy calculations have shown that isotactic ECC chains assume nearly extended conformations in the crystalline state, having ti or s(2/1)m symmetry according to the experimental chain axis periodicity. A high degree of conformational disorder is present in the crystalline state related to the conformational freedom of the cyclopentene rings which may assume twist (C-2-symmetric) and envelope (C-s-symmetric) conformations interconverting with a low-energy barrier. The conformation of the chains in the crystalline state is of kind (T-3 theta T-3-theta)(n) with theta variable in the range -50 degrees to +50 degrees, depending on the conformation of the five-membered rings, and corresponds to the repetition of anticlined enantiomorphic structural units along the chain. ECC represents a rare example of an isotactic polymer which assumes in the crystalline state a conformation that does not correspond to a helical repetition of isoclinic, isomorphous structural units. Depending on the succession of configurations (RS)(n) or (SR)(n) of tertiary carbon atoms in consecutive cyclopentene rings, the polymer chains may assume Z- or S-shapes. Chains in ti or s(2/1)m conformation are packed in an orthorhombic unit cell with axes a = 7.83 angstrom, b = 8.76 angstrom, and c = 9.0 angstrom. Dynamic conformational disorder and packing disorder due to the statistical substitution of chains having S- and Z-shapes are present. The structure may be described by statistical space groups Pcan and/or Ccmm for ti and s(2/1)m conformations, respectively.