The interest in the physics and chemistry of natural silk is due to the extraordinary mechanical, chemical, and biological properties and high functionality of this material. In particular, its unique mechanical behavior, which has never been realized in man-made fibers so far, makes the silk so attractive for materials scientists. We report here the occurrence of pure cyclic contraction in fibroin nanofibers electrospun from regenerated Bombyx mori silk. Evidence is presented of the essential role the oriented noncrystalline molecular chains and the water-soluble silk-I crystal modification play in this reversible mechanical process. We also show that regenerated silk may permanently supercontract if it contains a minimum amount of silk-II beta-crystals.