In the absence of oxygen, a strain of sarcina ventriculi, isolated from soil, could rapidly and completely ferment up to 20 g/l of arabinose. The principal products were ethanol, acetate, CO2 and H2. The yield of alcohol, up to 30% by weight of the sugar fermented, was not appreciably influenced by the pH of fermentation in the range 4–7. Sugar concentrations up to 100 g/l did not affect initial growth, but fermentation was incomplete at high sugar levels. This was probably due to the accumulation of end products other than ethanol, because the cells could grow in the presence of up to 25 g/l of added ethanol. Glucose, galactose and arabinose were sequentially utilized, in that order, when initially present as a mixed substrate. These sugars are major components of the hemicellulose from some agricultural residues. Practical implications for the general problem of pentose conversion to alcohol are discussed briefly.