AN interpolymer of ethylene and vinyl alcohol (prepared by co-polymerization of vinyl acetate and ethylene followed by hydrolysis1) has been found to give a crystalline X-ray diffraction pattern different from those of either polythene or polyvinyl alcohol. The fibre repeat distance is 2.5 A., corresponding to a single hydrocarbon chain zigzag. This fact alone indicates that there is no regular succession of the different monomer units along the chain molecules, because such a regular arrangement would have a fibre repeat period of at least twice the observed value. The succession of monomer units is thus apparently random, so that this is an example of the crystallization of high-polymer molecules of geometrically irregular structure. Such a crystal can be appropriately regarded as a mixed crystal. We have here yet another analogy between the crystallography of polymeric substances and that of monomeric chemical compounds.