Structural changes during tensile deformation in semicrystalline polymers are studied by the analysis of both small- and wide-angle X-ray scattering data from polyamide-6 fibers. The strain in the lamellar spacing is about the same as or higher than the fiber strain, suggesting that fiber elongation occurs by the deformation of the lamellar stack rather than slippage of the fibrils, especially during initial stages of elongation. The modulus of the lamellar structure is approximately 5 GN/m(2), and this is close to the fiber modulus, which is only 2-3% of the crystal modulus. Fiber modulus is, therefore, determined by the lamellar structure as much as by the interfibrillar oriented chain segments. The four-point small-angle X-ray scattering pattern in the relaxed fiber transforms reversibly into a two-point pattern under strain. The structures that correspond to these two patterns, the bistable states of the lamellae, coexist until fiber breakage. The structure that gives rise to the two-point pattern determines the ultimate strength of the fiber. Despite the small crystalline strain in the fiber direction, it is possible to follow the almost fully reversible changes in the orientation, size, and unit cell of the lamellar crystals. It is proposed that the appearance of the two-point pattern, the decrease in the lateral crystallite size, and the increase in the stack diameter are due to tilting of the lamellar surface caused by large-scale reversible strain in the interlamellar amorphous regions. This tilt is accomplished by slippage of the hydrogen-bonded sheets along the chain axis.