The effect of water on regenerated silkworm silk fibers has been studied and compared with that of water on natural silkworm silk fibers. Regenerated fibers are spun from an N-methylmorpholine-N-oxide (NMMO) fibroin solution through a wet-spinning process, leading to fibers with two distinct tensile behaviors, labeled as brittle and ductile, respectively. Regenerated fibers show a significant contraction when immersed in water. Contraction increases further after drying. In contrast, natural silkworm silk fibers show a negligible contraction when submerged in water. Regenerated fibers tested in water are considerably more compliant than samples tested in air, though their stiffness and tensile strength are significantly reduced. It has been shown that the tensile properties of brittle regenerated fibers can be modified by a wet-stretching process, which consists of deforming the fiber while immersed in water. Regenerated wet-stretched fibers always show a ductile behavior independent from their initial tensile behavior. (C) 2008 Wiley Periodicals, Inc.