In an apple juice-based medium, an ethanol-tolerant Australian wine-yeast used for cider manufacture produced more than 10% ethanol over a 5 week period. Growth of the inoculum (10 6 organisms ml 1 ) occurred to a population of 3โข1 10 7 ml 1 during the first few days; at the end of the fermentation only 5 10 5 yeasts ml 1 could be recovered as colony-forming units on plates. Respiratory and fermentative activities were measured by mass spectrometric measurements (O 2 consumption and CO 2 and ethanol production) of washed yeast suspensions taken from the cider fermentation at intervals. Both endogenous and glucose-supported energy-yielding metabolism declined, especially during the first 20 days. Levels of adenine nucleotides also showed decreases after day 1, as did adenylate energy charge, although in a prolonged (16โข5 week) fermentation the lowest value calculated was 0โข55. AMP was released into the medium. 31 P-NMR spectra showed that by comparison with aerobically grown yeast, that from the later stages of the cider fermentation showed little polyphosphate. However, as previously concluded from studies of 'acidification power' and fluorescent oxonol dye exclusion , repitching of yeast indicated little loss of viability despite considerable loss of vitality.