Cobalt-doped ZnO (Zn1-xCoxO) thin films were fabricated by reactive magnetron cosputtering. The processing conditions were carefully designed to avoid the occurrence of Co precipitations. The films are c-axis oriented, and the solubility limit of Co in ZnO is less than 17%, determined by x-ray diffraction. X-ray photoemission spectroscopy measurements show Co ions have a chemical valance of 2+. In this paper, hysteresis loops were clearly observed for Zn1-xCoxO films at room temperature. The coercive field, as well as saturation magnetization per Co atom, decreases with increasing Co content, within the range of 0.07≪x≪0.17. Most intriguing, the Zn1-xCoxO films are nonconductive as x is no more than 17%. Our results clearly demonstrate that ferromagnetism can be realized in Zn1-xCoxO without carrier incorporation. © 2004 American Institute of Physics.