The question of how carbon nanotubes (CNT)—believed to be the strongest filaments—relax under tension has been addressed. A dislocation theory applied to a two-dimensional nanocrystal such as the CNT describes the main routes of mechanical relaxation in this molecular structure: a brittle cleavage or, at high temperatures, a plastic flow. Both start with diatomic rotation, which “unlocks” the pristine wall of CNT by creating a dislocation dipole with the pentagon–heptagon cores. Under high stress, the dislocations depart from each other along helical paths, leaving behind a nanotube of smaller diameter, well-defined new symmetry, and changed electrical properties. © 1998 American Institute of Physics.