The invention relates to a fixing thread 1, 5, 10, 16 for sewing together reinforcing fibers, in particular to form a fiber preform, for the production of a fiber-reinforced composite component by means of impregnation with a curable polymer material.According to the invention, the fixing thread 1, 5, 10, 16 has an interface 4, 9, 15, 19 for locally increasing the ductility of a matrix formed by the curable polymer material and has at least one core thread 2, 6, 11, 12, 17, so as in particular to reduce the tendency for microcracks to form in the matrix of the finished composite component in the regions where the fixing threads 1, 5, 10, 16 penetrate.The interface 4 may be formed, for example, by an enclosure of the core thread 1 by a thermoplastic polymer material, for example by polyethylene, polypropylene or the like.