The monoclinic → cubic transformation in ZrO2, induced by fission-fragment irradiation, has been studied in single crystals. The transformed regions are partly coherent with the monoclinic matrix, as revealed by x-ray diffraction and electron microscopy. The transformation would seem to be due to stabilization of cubic zirconia with anion vacancies, produced by atomic collisions at the end of the fission fragment trajectory, rather than a “thermal spike” mechanism.