Candida sake was cultured on nutrient yeast dextrose broth, which was diluted and/or modified by the addition of either glycerol, glucose to 0·96 or trehalose to 0·97 water activity (aw) to modify endogenous sugar alcohol and sugar content. Sugar alcohols (glycerol, erythritol, arabitol and mannitol) and sugars (trehalose and glucose) were extracted from the yeast cells and quantified using HPLC. Total polyol and sugar content varied significantly between treatments. The total intracellular concentrations in NYDB medium were significantly increased in NYDB+glucose media. The major intracellular polyols/sugars in cells grown on unmodified NYDB were arabitol, trehalose and glucose with small amounts of glycerol and erythritol. This was changed by reducing aw of the growth medium, particularly with glucose or glycerol. The major polyols in C. sake cells grown on glucose-modified media were arabitol and the low mol wt polyol glycerol, with smaller amounts of glucose. In glycerol-amended full strength normal and diluted media, glycerol was the major intracellular polyol with lower amounts of the other polyols and sugars. The viability of the yeast cells with modified polyols/sugars was significantly improved at lowered aw level (0·935 and 0·95) when compared to unmodified yeast cells. Improvements in water stress tolerance was better in yeast cells grown for 48 than 24 h, especially in those from NYDB modified with either glucose, glycerol or trehalose. Such modifications of endogenous reserves were also shown to preserve or improve the biocontrol potential of the yeast against Penicillium expansum rot of apples. Thus physiological manipulation of intracellular polyol and sugar content may provide a means for developing biocontrol agents with improved ecological fitness in field environments.