Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to examine the nanostructure of a natural polymer-a spider dragline silk-that has potential applications as an engineering material. The silk studied was collected from the cob-web weaving spider Latrodectus hesperus. Single crystal and polycrystalline electron diffraction patterns indicate the presence of crystals with a bimodal size distribution, in the range of 2 nm and 40-120 nm. The chain axis of the smaller crystals is more strictly aligned with the fiber axis than that of the larger crystals. Lattice parameters for the orthogonal unit cell are: a = 9.4 angstrom (interchain), b = 7.0 angstrom (dipeptide, fiber axis) and c = 10.8 angstrom (intersheet). A fine structure in single crystal electron diffraction pattems indicates possible composition-dependent lattice strains. Results of tensile tests of the spider dragline silk are reported, and a simple model is presented linking the observed nanostructural features to the force-elongation response of this material. (c) 2006 Elsevier Ltd. All rights reserved.