The degree of crystal perfection in semicrystalline poly(ethy1ene terephthalate) (PET) fibers and films, produced under different conditions, was estimated in terms of the Ruland-Vonk crystal disorder parameter with the help of x-ray diffraction studies. Fibers and films which were produced by low-speed melt extrusion followed by drawing in water at low strain rates in a low-temperature environment had the lowest x-ray crystallinity and the most imperfect crystals. Further evidence of greater crystalline imperfection in these samples was provided by their x-ray intensity plots and melting endotherms. The low x-ray crystallinity and high defect density shown by these samples have been attributed to (a) the slow drawing at relatively low temperature, which does not provide the necessary environment for crystallization; and (b) the presence of very small nuclei which may be crystalline remnants that are retained at the low speeds of production and which can act as seed crystals around which crystallization occurs with incorporation of defects. On heat setting of these samples at elevated temperatures, crystal perfection improves significantly. On the other hand, samples produced by drawing at a relatively higher effective temperature and high velocity have higher crystallinity and more perfect crystals which crystallize further on heat setting without any significant enhancement of crystal perfection, except at very high temperatures.