Abstract: Improvements in polyester yarns of high strength from high viscosity polymer, obtained by melt-spinning such high viscosity polymer at high withdrawal speed to provide a novel intermediate partially-oriented yarn of high viscosity polymer, followed by drawing such intermediate partially-oriented yarn at the appropriate draw ratio to provide the desired high strength polyester yarn, especially of fine denier per filament to provide surprising advantages in durability as shown by an increased flex life, and if desired using a warp-drawing process to provide warp yarns of such high strength polyester, or a draw-texturing process to produce draw-textured yarns.