Reversible 300 K ferromagnetic ordering in a diluted magnetic semiconductor. Co2+:ZnO, is achieved by lattice incorporation and removal of the native n-type defect, interstitial Zn, resulting in "off" and "on" states (Figure. red triangles and blue circles). Spectroscopic and magnetic data implicate a double-exchange mechanism for ferromagnetism. These results suggest new opportunities for integrating, magnetism and conductivity in semiconductor sensor or spin-based electronics devices.