ZnO:In films were prepared by a sputtering method with targets facing. The lowest resistivity film is obtained at a substrate temperature of 150 degrees C using a ZnO target doped with 3 wt % In2O3. At substrate temperatures above 300 degrees C the resistivity of the film increases as the carrier concentration decreases. This implies a significant decrease in the donor impurity, ascribed to evaporation of the indium during film growth. From experiments with an additional Zn supply during deposition, we found that all of the doped indium is ionized as donor in ZnO:In, and that evaporation of depositing indium atoms is enhanced above 250 degrees C.