Films of the chalcopyrite semiconductor CuInS2 were grown by rf ion plating at low substrate temperature of 400 degrees C for various levels of substrate bias, ranging from +50 to -50 V. The Cu and In compositions were controlled by varying the electron beam power of the Cu2S and In2S3 sources. The crystalline quality was influenced by the level of the substrate bias. There were significant differences in surface morphology and crystallinity between films prepared under either negatively biased or floating conditions and those prepared under either positively biased or grounded conditions, Single-phase CuInS2 films of the best quality were obtained when the substrate was subjected to the floating condition. Such substrate bias dependence of crystalline quality clearly demonstrates the critical role that ions play in films prepared at low temperature.