Models for the crystal structure of alternating isotactic ethylene-norbornene copolymers (ENCs) are presented. Oriented fibers of ENCs samples have been obtained, and the.corresponding X-ray fiber diffraction pattern has been reported. A value of the chain axis periodicity of 8.9 Angstrom has been evaluated. A geometrical analysis and conformational energy calculations have shown that both isotactic and syndiotactic ENC chains assume nearly extended conformations having 2-fold helical s(2/1)m and glide plane tcm symmetries, respectively. Both conformations account for the experimental chain axis periodicity of 8.9 Angstrom. The crystal structure is mainly defined by the packing of the quasi-spherical norbornene units. The barycenters of the norbornene units are arranged on a face-centered crystalline lattice, producing a short-range three-dimensional positional order. Different kinds of structural disorder are present in the structure. A short-range order in the average positioning of the norbornene rings is maintained, while disorder in the positioning of the carbon atoms of the ethylene units is present. The ethylene units may assume different positions along a and b axes of the unit cell and connect with equal probability a given norbornene unit with any of its next neighbors, producing orientational disorder of the polymer chains as well as of the spherical norbornene units. The amount and the kind of disorder depend on the microstructure of the chains and on the condition of crystallization. The structure may be described by limit disordered models of packing, in unit cells having a = b = 9.4 Angstrom and c = 8.9 Angstrom or a = 9.3 Angstrom, b = 9.5 Angstrom, and c = 8.9 Angstrom. and statistical orthorhombic or tetragonal space groups Bmcm or I4/mmm. The partial threedimensional order, guided by the ordered positioning of the ball-like norbornene units, is obtained even though the polymers are configurationally disordered, provided that they have a regular alternation of the comonomeric units.