Drinking of excessive ethanol during pregnancy induces a fetal alcohol syndrome. Genistein is one of naturally occurring isoflavones at relatively high levels in soybeans. In this study, we investigated the effects of genistein ($1{ imes}10^{-8}$ and $1{ imes}10^{-7};{mu}g$/ml) on the ethanol (1 ${mu}l$/ml)-induced teratogenesis of developing mouse embryos during the critical period (embryonic days 8.5~10.5) of organogenesis using a whole embryo culture system and then morphological scoring analysis. Ethanol-treated embryos exhibited a variety of developmental abnormalities. However, the total morphological scores for ethanol plus genistein groups were significantly higher than those of ethanol alone group (p<0.05). In particular, there were significant increases in the ethanol plus $1{ imes}10^{-8};{mu}g$/ml of genistein group on the scores for heart, optic system, branchial bar, mandibular process, and caudal neural tube and further in the ethanol plus $1{ imes}10^{-7};{mu}g$/ml of genistein group on the scores for heart, hind-, mid-, and forebrains, optic system, branchial bars, maxillary and mandibular processes, caudal neural tube, forelimb, hindlimb, and somites as compared with those of ethanol alone group (p<0.05). These results indicate that genistein has a preventive effect against ethanol-induced teratogenesis.