Yeast strains were examined for their ability to produce glycerol, and Kluyveromyces marxianus NRRLY-665 was chosen for further optimization due to its comparatively high growth rate (0.15 h(-1)) and glycerol yield (9.5%) by weight on lactose) at 37 degrees C and pH 7. Optimal conditions for glycerol formation were 30-37 degrees C and pH 6-7. Increasing whey permeate concentration above the equivalent of 4.5% lactose decreased growth rate and cell production and increased acetic acid production. When sulfite was added throughout the fermentation to complex acetaldehyde before it could be converted to ethanol, 21% glycerol yield was obtained at 4.5% g l(-1) free sulfate, pH 7, 20 degrees C, and <50 mm Hg pressure, but at a much lower growth rate than in the absence of sulfite at 37 degrees C. Several osmotolerant yeast stains were cultivated on regular and enzyme-hydrolyzed whey permeate at high NaCl concentrations. One of them, Pichia fariniosa DSM 70362, did not produce glycerol from whey permeate but gave 4% yield on enzyme-hydrolyzed lactose. Osmotolerant yeast metabolized glycerol after glucose, at the same time as galactose, and before lactose.