Ethylene-enhanced leaf elongation upon submergence is part of the survival mechanism of Rumex palustris Sm. plants that grow in frequently flooded areas. Other Rumex species, like R. acetosa L., do not possess this ability and can therefore only survive in habitats that are not frequently inundated. Expansins are proteins that induce extension of isolated cell walls, and therefore might play a role in the stimulation of petiole elongation, also in Rumex. We report here on the identification of several gene sequences encoding for alpha-expansins in R. palustris and R. acetosa plants. The pattern of transcript accumulation of one of these genes, Rp-EXP1, could be correlated with the pattern of leaf elongation in R. palustris after submergence or ethylene treatment. Induction of expansin gene activity was not found in R. acetosa upon these treatments, indicating that ethylene induces the expression of expansin genes in leaves of species that exhibit flooding-induced shoot elongation.