To understand the mechanism of ion homeostasis in salt tolerant and sensitive plants, we isolated cDNAs for K(+) transporter PhaHAK1-u and PhaHAK5-u from reed plants. PhaHAK1-u belongs to group I and PhaHAK5-u belongs to group IV by phylogenetic analysis, respectively. PhaHAK5-u is predicted to be a plasma membrane transporter, and shows high-affinity K(+) transporter. Expression of PhaHAK5 was found in salt-sensitive reed plants, but not in any parts of salt-tolerant reed plants maintained under both control and K(+) starvation conditions. Under the NaCl stress, the K(+) uptake ability of the yeast strain expressing PhaHAK5-u was remarkably lower than that of the yeast strain expressing PhaHAK1-u, and PhaHAK5-u showed Na(+) permeability. These results suggest that PhaHAK5 is one of the routes by which Na(+) enters cells.