PMA1 and PMA2 genes encode Saccharomyces cerevisiae plasma membrane H+-ATPase (PM-H+-ATPase), an enzyme with critical physiological roles both in the absence or presence of environmental stress. The two PM-H+-ATPase isoforms differ in their biochemical characteristics but, under all the growth conditions that were examined so far, PMA2 expression is negligible and Pma1p accounts for practically the totality of cell PM-H+-ATPase activity. In the present work, we have compared gene expression levels and activity of this proton pump in yeast cells cultivated under fermentative or respiratory growth and under carbon starvation. The expression levels of both PMA1 and PMA2 genes were consistently higher (2.5–4.5-fold) in cells cultivated under respiratory metabolism (in ethanol-based medium or after the diauxic shift), than in cells cultivated under fermentative metabolism (during the full period of growth in a medium where glucose is not the limiting nutrient or during the first period of diauxic growth in low-glucose-based medium). The moderate upregulation of PMA1 and PMA2 transcription in cells grown on ethanol compared with those grown on glucose was reflected in the increased content and activity of PM-H+-ATPase. In diauxic growth, during transition to stationary phase after ethanol depletion, a further strong activation (eight-fold) of PMA2 gene transcription was observed. Although PMA2 transcription still remains quite below (20-fold) PMA1 transcription, this is the first environmental condition, identified so far, that leads to a significant PMA2 expression, suggesting that this PM-H+-ATPase isoform may play some role during carbon starvation. Copyright © 2003 John Wiley & Sons, Ltd.