The morphological structure of an extremely highly crystalline poly(ethylene terephthalate) (PET) which was obtained by very long term annealing after direct esterification was characterized by small angle and wide angle X-ray diffraction, infra-red spectroscopy, and density and fusion measurements. The specimen has a high melting point of 302.5 degrees C, an observed density of 1.468 g cm(-3) and a calculated density of 1.501 g cm(-3). From these results, together with small angle X-ray scattering data and the dependence of the fusion curves on the heating rate, this highly crystalline material is proposed to consist of molecules which are either fully extended or which contain only a few folds. Moreover, the specimen has a good double orientation : the crystallites not only have their a axes parallel, but also have their (001) planes approximately parallel to the main plane of the specimen. The structure determined from the X-ray diffraction intensity data is basically consistent with that obtained by Daubeny et al.