We have used a hydrothermal technique to synthesize polycrystalline Zn-1-xCoxO diluted magnetic semiconductor. The synthesis temperature was of 240degreesC and the Co concentration of 10 at.%. X-ray diffraction indicates that the Co-doped ZnO thus prepared has the pure ZnO wurtzite structure without any significant change in the cell parameters. Optical absorption measurements show several absorption bands that are due to the transitions between the crystal-field-split 3d levels of tetrahedral Co2+ substituting Zn2+ ions. Room temperature magnetization measurements reveal a paramagnetic behavior for the Co-doped ZnO samples. This is attributed to the absence of free carriers. (C) 2004 Elsevier B.V. All rights reserved.