The yeasts Saccharomyces rouxii (xerotolerant strain) and S. cerevisiae (non-tolerant strain) were grown in media adjusted to the specified water activity with NaCl or polyethylene glycol. Polyol production (intracellular and extracellular glycerol and arabitol) in S. rouxii was studied as a function of water activity adjusted with polyethylene glycol. The essence of the response of S. rouxii to lowered water activity was to retain within the cell an increasing proportion of glycerol. Arabitol was not appreciably affected by water activity. S. cerevisiae responded to diminishing water activity by synthesizing much more glycerol but retaining a lower and less variable proportion of it. The response of S. rouxii was at the level of glycerol permeation/transport while that of S. cerevisiae was metabolic.