Cosmetic composition comprising rigid, substantially rectilinear fibers of a synthetic polymer in a physiologically acceptable medium, in which the polymer is chosen from polyurethanes, polyesters, acrylic polymers, polyolefins, non-aromatic polyamides and aromatic polyimide-amides.The composition may be a makeup composition, a makeup base, a composition to be applied over a makeup, also known as a “top coat”, or a cosmetic treatment or care composition for keratin fibres. The invention relates more particularly to a mascara.The invention also relates to the use of this composition for making up keratin materials, especially the eyelashes, the eyebrows and the hair, and also to a cosmetic makeup or care process for these materials.