A method is described for qualitatively monitoring changes in the starch gel electrophoresis isozyme patterns of key yeast dehydrogenase and reductase activities involved in the transition from aerobic to anaerobic conditions during batch fermentations. The hypoxic transition was accompanied by the disappearance of catalase, superoxide dismutase, mitochondrial dehydrogenases and the appearance of a novel 6-phosphogluconic acid dehydrogenase activity. Significant genetic differences were noted in the banding patterns of wine yeasts, baker's yeast and a commonly used laboratory yeast strain.