1.The organic acid metabolism of the roots of wet-land species (helophytes) and that of dry-land species (non-helophytes) has been examined in relation to their tolerance to periods of experimental flooding. Growth differences between helophytes and non-helophytes were apparent only after an 18-day flood period, yet within four days of flooding differences could be observed in the levels of certain organic acids. Flooding in helophytes increased the level of root malic acid, and decreased that of succinic and lactic acid, whereas the reverse was found in non-helophytes. 2. There was evidence that under natural flooded conditions the root tissues of some wet-land species contain larger amounts of malic acid than when the flood water has receded and ground aeration improves. Shikimic acid has been detected in aquatic macrophytes and the fluctuating levels of shikimate in Iris pseudacorus and Nuphar lutea are discussed. 3. The organic acid changes related to flood tolerance operate only under partial anoxia, and under the strictly anaerobic conditions imposed by incubation of root tissue under nitrogen, there was a general reduction in all acid levels except lactate. It is suggested that non-helophytes cannot tolerate flooding through an inability to continue TCA cycle respiration during periods of reduced oxygen supply, and through the poisoning effects of ethanol accumulation. Helophyte species appear to be metabolically adapted to overcome periods of flood-induced anoxia, and tissue respiration continues with the provision for malic acid accumulation and no build-up of ethanol. 5. A tolerance of flooding, involving adaptations of the organic acid metabolism of the roots, has been demonstrated in helophyte species. This metabolic adaptation is rapidly induced, thus offering immediate protection upon flooding anoxia, and its importance in determining the ecological amplitude of a species and its possible role in future production of flood tolerant strains, are discussed.