Many yeasts can aerobically catabolize exogenously supplied glycosides that are hydrolysed in the cytosol, but few do so anaerobically. This is so, even for yeasts that use one or more of the component hexoses anaerobically. The phenomenon, called the Kluyver effect, appears to be brought about by a combination of the following four factors: (i) fast transport of the glycosides into the cells involves proton symport and seems to require aerobiosis, so, under anaerobic conditions, the glycosides enter the cells much more slowly. This is probably because there is less ATP produced anaerobically than aerobically and, consequently, insufficient to supply the proton pump optimally, which is necessary to maintain proton symport; (ii) in addition, anaerobically, the transport carrier may have a lower substrate affinity; (iii) glycosidases generally have low substrate affinities; and (iv) the consequence of (i), (ii) and (iii) is a lowering of glycolytic flux and this deactivates pyruvate decarboxylase.