THE level of the Dead Sea, the lowest (about -400 m) and one of the most salty (salinity about 340 g l(-1))lakes on earth, is lowering at a rate of approximately 0.5 m annually owing to extensive exploitation of its main perennial tributary (the Jordan River) and the extreme aridity of the region (annual precipitation is about 60 mm)(1). Consequently, new hypersaline sea shores are exposed, forming a unique, originally sterile ecosystem. The first plants invade these newly exposed shores after several years while soil water salinity is still extremely high, Here we use stable isotopes of oxygen and hydrogen to show that a variety of such perennial pioneer plants are able to make use of occasional floodwater which is distinct from the bulk of the hypersaline soil water found in their root zone. Our results provide new insight into the ways in which plants can invade extremely hostile environments and extend their ecological limits of distribution.