A binding fiber having, as at least one first thermoplastic component, a copolyamide which has a relative viscosity of at least 1.50 (measured at 0.5% in m-cresol at 25° C.), a MVR of maximum 20 cm3/10 min at 190° C. and 2.16 kg load, a hydrophobicity of less than 7.2, a melting temperature lower than 105° C. in the water saturated condition and a higher melting temperature in the dry condition. The binding fiber can, on the one hand, be activated with hot steam at approximately ambient pressure during the manufacture of flat materials containing natural fibers, such as paper, cardboard, wood fiber boards, or fiber mats. The finished flat material can, on the other hand, be subjected to higher temperatures, and the copolyamide develops good bonding with respect to the cellulose fibers normally used in the indicated flat materials.