Abstract: Provided is a process for forming and modifying a self-crimping polyester yarn comprising a plurality of polyester filaments having thick and thin regions along their lengths which are out of phase from filament to filament. The plurality of filaments are produced by first forming a plurality of combined streams of molten melt spun polyester by combining at least first and second extruded molten streams of fiber forming polyester travelling at different extrusion speeds to thereby form thick and thin regions in the combined streams out of phase from other combined streams. The combined streams are then quenched and thereby transformed into solid filaments in a solidification zone provided with a gaseous atmosphere at a temperature below the glass transition temperature of the polyester.