The pma2 gene of Schizosaccharomyces pombe codes for a polypeptide having a predicted Mr of 110,126 and which is 79% identical to the plasma membrane H(+)-ATPase encoded by the pma1 gene. The pma2 gene, unlike pma1, is weakly expressed and not essential to mitotic growth. By constructing yeast strains in which the chromosomal pma2 gene is under control of the adh promoter, it has been possible to identify the overproduced ATPase in plasma membrane via formation of a phosphoenzyme. In a pma1-1 mutant strain whose ATPase activity is insensitive to vanadate, the overexpressed pma2 gene restores vanadate sensitivity. It also rescues a pma1 null mutant from lethality. These results demonstrate that the two H(+)-ATPases are functionally interchangeable in vivo but differently expressed.