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Reed plants (Phragmites australis Trinius) grow not only in fresh and brackish water areas but also in arid and high salinity regions. Reed plants obtained from a riverside (Utsunomiya) were damaged by 257 mM NaCl, whereas desert plants (Nanpi) were not. When the plants were grown under salt stress, the shoots of the Utsunomiya plants contained high levels of sodium and low levels of potassium, whereas the upper part of the Nanpi plants contained low levels of sodium and high levels of potassium. One month salt stress did not affect potassium contents in either Utsunomiya or Nanpi plants, but it did dramatically increase sodium contents only in the Utsunomiya plants. The ratio of K+ to Na+ was maintained at a high level in the upper parts of the Nanpi plants, whereas the ratio markedly decreased in the Utsunomiya plants in the presence of NaCl. Accumulation of Na+ in the roots and Na+ efflux from the roots were greater in the Nanpi plants than in the Utsunomiya plants. These results suggest that the salt tolerance mechanisms of Nanpi reed plants include an improved ability to take up K+ to prevent an influx of Na+ and an improved ability to exclude Na+ from the roots.