It is generally admitted that, due to the low levels Of respiratory enzymes and mitochondrial cytochromes, classical oxidative phosphorylation does not occur to any significant extent under alcoholic fermentation conditions. Under anaerobiosis, yeast growth normally requires oxygen in order to favour the synthesis of sterols and unsaturated fatty acids. However, in such conditions, superfluous oxygen consumption by yeast cells is observed. In this study, the potential relationship between this oxygen consumption and the known respiratory pathways of S. cerevisiae was investigated during enological fermentation. Partial and limited synthesis of mitochondrial cytochromes was observed during the growth phase. The superfluous oxygen consumed by the yeast cells appeared not to be related to classical respiration, but mainly to the operation of several mitochondrial alternative respiration pathways, which were linked to the cell cytochrome contents. It was further found that oxygen-dependent ergosterol biosynthesis accounted for less than 15% of the total oxygen consumption at the beginning of stationary phase in the absence of anaerobic growth factors. No significant oxygen consumption could be clearly attributed to fatty acids desaturation in the yeast cells.