Hybrid yarns of PEEK and Carbon fibres are attractive as reinforcing materials. In an optimized hybrid yarn PEEK and Carbon fibres possess similar diameters. But the melt spinning of fine and ultra-fine thermoplastic filaments is a processing technique which makes great demands on the polymer and its elongational flow. The paper deals with theoretical and experimental investigations of the melt spinning process of fine and ultra-fine PEEK filaments up to finenesses of 1 dtex and lower. With a special spinning equipment finest filaments of lower than 0.1 dtex were reached. (C) 2003 Kluwer Academic Publishers.