The properties of PET (polyethylene terephthalate) fibers spun at high WUS (wind-up speed) have been compared with fibers drawn from low-WUS feedstock by measurements of initial modulus, birefringence, and extension to-break. Contrary to expectations, spun fibers are not like partially drawn fibers but are structurally different. Lack of appreciation of this can lead to errors when calibrations are used to characterize spun fibers which are based on the properties of drawn fibers (e.g., sonic modulus). Deformation and heat treatment of PET fibers leads to two types of crystallinity, strain induced (type I) and thermally induced (type II). Type I is due to extension of “backbone” molecules, and type II arises from epitaxial crystallization during heat treatment at constant length. In normal drawing type I is associated with extension on the hot feed roll; and type II, with the hot plate. In spinning at high WUS, type II occurs extensively at the crystallization WUS of about 4000 mpm, and thereafter type I and II occur together. When spun fibers are annealed taut there is a considerable increase in type II epitaxial crystallinity, and the difference between spun and drawn fibers can no longer be distinguished according to the criteria used.