Atomic force microscopy was used to study the three-dimensional nanometer-scale structure of the dragline silk of Nephila clavipes from microtomed sections of the silk. Contrary to a previously proposed model of randomly distributed protein crystallites interspersed in amorphous regions, a highly organized skin-core structure of the fiber was observed. The skin appeared to be thin with no discernible distinct features. The core consists of pleated fibril-like structures, which are arranged in two concentric cylinders. Upon stretching, the pleats were smoothed out substantially. The mechanical properties of spider silk can quite straightforwardly be related to the newly observed structures.