The rare aquatic plant Luronium natans (L.) Rafin. is a representative of the Littorelletea, i.e. plant communities growing typically in unproductive habitats with oligo- to meso-trophic, slightly acid to circumneutral soft waters. Apart from morphological and physiological adaptations to these particular habitat conditions, Littorelletea communities are characterised by their narrow ecological amplitude. Despite the fact that L. natans is reported to be limited to habitats with low calcium content, it is abundant in two of the numerous potential yet calcareous habitats in the floodplains of the Upper Rhone, and its tributary, the Ain River (France). The present study examines whether the restriction to such a low number of habitats is linked to particular abiotic conditions therein. It also aims to identify the range of floodplain habitats that belong to the species' fundamental niche, i.e. those where, after arrival, it would be able to establish in the absence of interspecific competition. L. natans was therefore transplanted to sites along a gradient of sediment composition, and allowed to establish during a competition-free period. Extensive physico-chemical analyses were carried out on sediment, surface and interstitial water of the transplantation sites. The species' fundamental niche appeared very large, since establishment was successful in 3/4 of the transplantation sites. The magnitude of establishment success differed greatly, but only redox-potential was significantly correlated with establishment success. Abiotic parameters could not provide a general explanation of complete establishment failure either. The experiment demonstrated that the restriction of L. natans distribution in these floodplains is not caused by a lack of habitats with suitable water and sediment chemistry. (C)2000 Elsevier Science B.V. All rights reserved.