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The conformation and the packing of the chains of the alternating ethylene-chlorotrifluoro-ethylene copolymer (ECTFE), in the crystalline phase, are investigated by X-ray diffraction analysis, by calculations of the conformational energy and by calculations of the Fourier-transforms of models. These analyses suggest that the crystalline form of ECTFE is a mesomorphic form, with a pseudo-hexagonal packing of the axes of nearly trans-planar chains (interchain distances of 5,69 A) and a nearly complete translational and rotational disorder along the chain axis.