Abstract: A mat useful for reinforcing composite materials is made up of at least first and second sets of fibers, the first set of fibers laid as a ply in spaced apart parallel relationship in the direction of construction and the second set laid as a ply in spaced apart parallel relationship perpendicular to the direction of construction. These fibers define interstices, into which a continuous filament is knitted to provide stability. The filament has a core portion and a sheath portion, the sheath portion having a lower melting point than the core portion. The sheath portion is thermally fusible to form a plurality of closed loops in which the respective sets of fibers are entrapped. The fibers are glass or glass rovings and the filament is a polyester, the core portion being a high-viscosity poly(ethylene terephthalate) and the sheath portion being a low-viscosity poly(ethylene terephthalate).