The surfaces of nylon 6, nylon 66, and polyethylene terephthalate fibers were modified by monomeric fluorine-containing additives. The additives were blended into the polymer melt prior to extrusion and were designed in such a way that they could migrate through the polymers and concentrate in the fiber matrix near the fiber surfaces. At the fiber surface a continuum of closely packed fluorocarbon chains is formed, which results in a fiber of low surface energy with good resistance to abrasion. The modified fiber can be dyed in a hot dyebath despite its strong water and oil repellency.