In higher plants, alcoholic fermentation is required to supply NAD⁺ to the glycolytic pathway, which is responsible for ATP synthesis under anaerobic conditions. Matsumura et al. (1995) previously isolated the reduced adh activity (rad) mutant in rice, in which elongation of the coleoptile is repressed under submergence. However, the rad gene had not been characterized. In the present study, we observed that, in the coleoptile, the Adh1 mRNA levels were comparable between the rad mutant and the wild type cultivar Kinmaze, while the amount of ADH1 protein was much lower in the rad mutant than in the wild type. Sequencing showed that G106 of the Adh1 gene of Kinmaze was replaced with A in the rad mutant, resulting in an E36K substitution in the deduced amino acid sequence. A genotyping experiment using F₂ plants from a cross of the rad mutant with the indica cultivar Kasalath indicated that the point mutation was involved in the repression of coleoptile elongation. Furthermore, since the reduced ADH activity appeared to cause an ATP deficiency, elongation of the coleoptile was repressed in the submerged rad mutant.