A method for impregnating strands or strips of natural fibers, using the following successive steps: i) the impregnation of the strands or strips by immersion in a bath containing a fine aqueous polymer dispersion; followed by ii) the drying of the strands or strips using a heating system, with the progressive elimination of the water and the gradual melting of the polymer, and the coating of the strands or strips and the molten polymer incorporated into the core of the strands or strips as a binder between the fibers; iii) optionally, the forming of the treated strands or strips into their final shape; and iv) the cooling of the treated strands or strips. The aqueous polymer dispersion comprises at least one semi-crystalline or amorphous polymer and, in the case of an amorphous polymer, has a Tg varying between 50° C. and 175° C.