Atomic force microscopy was used to study the internal structures of Kevlar(R) 49 fibers and spider dragline silk from the microtomed sections of the fibers. Both fibers exhibited fibrillar structures. For Kevlar(R), the fibrils were aligned along the fiber axial direction, tightly packed and braided. In addition, a prominent skin layer was present. For the dragline silk, pleated fibrils that were unfolded upon extension were observed in the core region. The skin appeared to be very thin. These structural features correspond well with the mechanical properties of these high-strength fibers.