Drawn multifilament poly(ethylene terephthalate) yam was heat-set isothermally at temperatures between 180 and 258 degrees C in the slack and taut conditions. The apparent crystal widths of these samples in the undeformed state and when axially stretched by 5-40% were monitored by X-ray diffraction. Significant reduction of crystal width occurred in the [010] direction in samples heat-set at high temperatures. Also, while this reduction in width commenced at low extension in the taut-annealed samples, it occurred at relatively higher extension in the slack annealed yarns. These differences in behaviour have been attributed to the differences in the mechanisms of deformation that become operative on stretching the two sets of samples, and their structural origins have been examined.