Transmission electron microscopy (TEM) at high magnification of thin cross-sections of drawn multifilament poly (ethylene terephthalate) yam and of this yarn after heat-setting in the slack state at 250 degrees C and in the taut state at 258 degrees C is shown to provide evidence for the existence of crystallites whose lateral dimensions are consistent with those measured from wide-angle X-ray diffraction (WAXD). The denser, crystalline microfibrils appear to be separated by interfibrillar material of lower density in all the samples examined. The electron micrographs bring out the important differences in the morphologies of the three samples, the implications of which are critically examined.