Pichia sorbitophila grows rapidly in the presence of very high NaCl concentrations. Under these conditions, even when the K+ concentration is low, P. sorbitophila cells can maintain low Na+ and high K+ contents. This remarkable capacity of P. sorbitophila fails when the external pH is not acidic. This indicates that Na+ efflux is mediated by an electroneutral Na+/H+ antiporter. We have cloned and sequenced two genes designated as PsNHA1 and PsNHA2, which probably encode two antiporters of this type. The genes present high similarity with the corresponding genes from other yeasts. The heterologous expression of PsNHA1 or PsNHA2 in a Saccharomyces cerevisiae mutant lacking the Na+ efflux systems and sensitive to high concentrations of Na+ and K+ rescued the tolerance and the ability to extrude both cations. The Accession Nos of the sequenced DNA fragments are: PsNHA1, AJ496431; PsNHA2, AJ496432. (TC 2.A.36) Copyright © 2002 John Wiley & Sons, Ltd.