Filaments having a substantially increased inner surface, in particular porous filaments of filaments having a cracked, notched or nicked surface of thermoplsatic material are produced by subjecting filaments of thermoplastic material, preferably containing a pore-forming agent, to a heating medium at a temperature at least 100.degree., preferably 150.degree.C. above the glass transition temperature of the thermoplastic material, preferably near or above its softening point, the temperature of the heating medium and the time of exposure of the filaments thereto being such that the peripheral portions of the filaments are affected by the heat treatment to a substantially larger degree than the core portions creating within the filaments temporarily a temperature gradient between the core and periphery of the filaments, the temperature at the periphery being sufficient to reduce the intermolecular cohesion of the thermoplastic material at the periphery, and the action of heat at the core being substantially lower than at the periphery.