Saccharomyces fragilis, a yeast capable of fermentation, grows poorly on ethanol as the carbon source. The respiratory quotient for glucose of aerobically grown cultures approaches 1 and glucose does not repress the formation of mitochondria. The effect of ethanol on the oxygen consumption and the NAD (nicotinamide adenine dinucleotide)-dependent alcohol dehydrogenase activity of this yeast has been studied. The electrophoretic pattern of alcohol dehydrogenase activity on polyacrylamide gels is complex. Six bands of activity can be identified in homogenates from cultures grown on glucose. Three of these appear associated with the particulate fraction. Differences in the relative amounts of the three particulate bands of activity under different growth conditions indicate that the particulate fraction may contain more than one isozyme. The particulate fraction also exhibits alcohol oxidase activity. While ethanol and acetate stimulate the oxygen consumption of the glucose-grown cultures, theQo2for these substrates is much less than that for glucose. If the glucose-grown cultures are transferred to fresh ethanol medium, theQo2for ethanol and acetate increases to values higher than that seen for glucose. This increase does not depend on an increase in the terminal respiratory capacity of the cells. On transfer to ethanol medium there is also a marked increase in the total alcohol dehydrogenase activity, and on electrophoresis a new band of activity appears. Acetate cannot replace ethanol in producing these effects.S.fragilisalso produces an NAD-dependent alcohol dehydrogenase which reacts with cinnamyl alcohol as substrate but not with ethanol.