High-affinity K+ transporters play an important role in K+ absorption of plants. We isolated a HAK gene from Aeluropus littoralis, a graminaceous halophyte. The amino acid sequence of AlHAK showed high homology with HAK transporters obtained from Oryza sativa (82%) and Hordeum vulgare (82%). When expressed in Saccharomyces cereviae W Delta 3, AlHAK performed high-affinity K+ uptake with a K-m value of 8 mu M, and the growth of transformants was dramatically inhibited by 150 mM Rb+ and 150 mM Cs+ but less affected by 300 mM Na+. AlHAK may thus improve the capacity of plants to maintain a high cytosolic K+/Na+ ratio at high salinity.