It is well documented that iodine kills microorganisms with a broad spectrum, but a systematic study of its mechanism of action has not yet been reported. Here we found the action of iodine on gene expression level, using the yeast Saccharomyces cerevisiae with a DNA microarray. It was found that, like antimicrobial activity, iodine causes an immediate and dose-dependent (0.5 mM, 0.75 mM and 1 mM) transcriptional alteration in yeast cells. The effects of iodine continued after the first immediate response. Genes for c-compound and carbohydrate metabolism, for energy, and for cell rescue were continuously up-regulated. On the other hand, genes related to protein fate were induced especially at 0.5 h. The gene expression profile at 0.5 h was significantly different from that of a longer iodine exposed condition. The main reaction at 0.5 h after iodine addition might be due to oxidative toxicity, and the profile at 0.5 h was similar to that of an agricultural bactericide.