The X-ray data for melt-spun fibres of the 33/33/33 copolyester prepared from p-hydroxybenzoic acid, biphenyl and terephthalic acid are characteristic of a completely random microstructure. Nevertheless, these copolymers adopt three-dimensionally ordered structures in the solid state, in which the chains are packed on hexagonal or orthorhombic polymorphic lattices. We have used molecular mechanics modelling to optimize the packing of random sequences in the higher-density orthorhombic form. The models consisted of 48 non-identical chains of nine monomers each. The random sequences were restricted to three monomers of each type, so that they had approximately the same length, making it possible to apply a periodic boundary condition. The initial model had the extended chains in register, i.e. their central ester oxygens were in a plane perpendicular to the chain axis direction. This structure had high potential energy due to overlap between the non-identical sequences. Energy minimization eliminated these bad contacts, at a cost of only similar to 1.8 kcal mol(-1) per monomer, by relatively small changes in the torsion angles at the phenylene-ester linkage bonds. These small arrays also predict Bragg maxima on the equator and layer lines that match those observed. We conclude that non-identical chains can be packed in a stereochemically acceptable manner according to the geometry defined by the X-ray data.