Both added (49.6 g/l) and produced ethanol (46.2 g/l) caused an increase in the acetic acid tolerance of Saccharomyces cerevisiae grown in an anaerobic chemostat; added ethanol, however, to a less extent than produced ethanol. The ethanol induced acetic acid tolerance of the cells was linked with an accumulation of trehalose within the cells. These results indicate that trehalose plays a role in the ethanol induced acetic acid tolerance of S. cerevisiae.