Natural rubber becomes crystalline when stretched or when cooled, yet the structure of this ordered form has never been satisfactorily determined. We have used film-recorded fiber X-ray diffraction data from oriented unvulcanized natural rubber, combined with linked-atom-least-squares molecular modeling and refinement, to provide what we believe is a better structure for crystalline natural rubber than any proposed before. Having tested other plausible alternatives, our final crystal structure is orthorhombic with P2(1)2(1)2(1) symmetry. The molecular ( and crystal) asymmetric unit is a di-isoprene in which the two isoprene residues have distinctive, not symmetrically related conformations. A significantly better fit with the Bragg diffraction pattern is obtained if it is assumed that sheets of rubber molecules can replace one another at random in the crystal structure. This structural feature also explains the non-Bragg layer line streaks that are a prominent feature of rubber diffraction patterns that hitherto have been largely ignored.