The role of ethylene in induction of aerenchyma formation and ethanolic fermentation in waterlogged roots of Dendranthema zawadskii and D. nankingense, two species that differ with respect to waterlogging tolerance, was examined. In the more tolerant D. zawadskii, but not in D. nankingense, ethylene accelerated programmed cell death and promoted formation of lysigenous aerenchyma, both of which were inhibited by treatment with the ethylene inhibitor 1-methylcyclopropene. Waterlogged D. zawadskii roots generated a higher quantity of endogenous ethylene than did those of D. nankingense. In waterlogged D. zawadskii roots, transcription of the genes encoding alcohol dehydrogenase (EC 1.1.1.1) and pyruvate decarboxylase (EC 4.1.1.1) increased rapidly but transiently, whereas expression of these genes in D. nankingense increased gradually and over a longer period. In D. nankingense, waterlogging elevated both alcohol dehydrogenase and pyruvate decarboxylase activity, and the production of ethanol and acetaldehyde was increased in the presence of exogenous ethylene and inhibited by 1-methylcyclopropene. In D. zawadskii, in contrast, after a prolonged episode of waterlogging stress, exogenous supply of ethylene suppressed the production of ethanol and acetaldehyde, whereas exogenous 1-methylcyclopropene enhanced their production. In the more tolerant Dendranthema species, ethylene appeared to signal an acceleration of both waterlogging-induced programmed cell death and aerenchyma formation and to alleviate ethanolic fermentation, whereas in the more sensitive species ethylene activated fermentation and increased the release of ethanol and acetaldehyde, which are by-products probably responsible for the collapse of the waterlogging-damaged root.