A novel ethanol-hypersensitive mutant, geko1 (gek1), was isolated from Arabidopsis thaliana. The gek1 mutant displays an enhanced sensitivity (10-100 times greater than the wild type) to ethanol in growth medium, while it grows normally in the absence of ethanol, and responds normally to other alcohols and to environmental stresses such as heat shock and high salinity. The ethanol-hypersensitive phenotype of gek1 requires alcohol dehydrogenase activity, indicating that gek1 is sensitive not to ethanol itself but to the metabolites of ethanol. Consistent with this, gek1 shows enhanced sensitivity to acetaldehyde in the medium. The endogenous acetaldehyde levels were not different between gek1-2 and wild-type seedlings treated with ethanol. These results indicate that the ethanol hypersensitivity of gek1 is due to an enhanced sensitivity to acetaldehyde toxicity, instead of abnormally elevated accumulation of toxic acetaldehyde, which has been thought to be the major cause of ethanol toxicity in mammal cells.