In high gravity Saccharomyces cerevisiae fermentations containing 300 g glucose l(-1), daily addition of acetaldehyde to a total of 93 mM shortened the time required to ferment the first 250 g glucose l(-1) from 790 h to 585 h. Acetaldehyde feeding had no effect on the ethanol yield but increased by 135%, 78% and 77% the final concentrations of 2,3-butanediol, 2-methylpropanol and acetate, while decreasing that of glycerol by 14%. Controlled acetaldehyde feeding has potential as a technique for accelerating high gravity fuel or industrial ethanol fermentations and may be useful in preventing incomplete fermentations.