Self-extinguishing filaments and fibers of modacryl polymers, wherein the filament-forming substance is a copolymer of from 35 to 85 weight % of acrylonitrile, from 5 to 45 weight % of vinyl chloride, vinyl bromide and/or vinylidene chloride, from 5 to 30 weight % of carboxyphosphinic acid derivatives of the formula ##STR11## where R.sub.1 is lower alkyl having up to 8 carbon atoms, or CH.sub.2 Cl--, R.sub.2 is lower alkyl having up to 8 carbon atoms in which a hydrogen atom may be substituted by a halogen atom in case of R.sub.1 being CH.sub.2 Cl--, and R.sub.3 is hydrogen or methyl, and from 0 to 15 weight % of other unsaturated compounds having an activated double bond;
- the weight percentages being relative to the total amount of monomers.