We isolated PhaHAK2 cDNAs from salt-tolerant and salt-sensitive reed plants. PhaHAK2 belongs to group II by phylogenetic analysis, and was predicted to be a high-affinity plasma membrane K+ transporter. Yeast transformed with the PhaHAK2-u from salt-sensitive reed plants (Phragmites australis) had a decreased ability to take up K+ in the presence of NaCl and showed a higher Na+ permeability than yeast transformed with PhaHAK2-n or PhaHAK2-e from two salt-tolerant reed plants. These results suggest a possibility that the continuous K+ uptake by PhaHAK2 and maintenance of high K+/Na+ ratio under salt stress condition is one of the causes of the salt-tolerance in reed plants.