The facultatively fermentative yeast Candida utilis exhibits the Kluyver effect for maltose: this disaccharide is respired and assimilated but, in contrast to glucose, it cannot be fermented. To study the mechanism of the Kluyver effect, metabolic responses of C. utilis to a transition from aerobic, sugar-limited growth to oxygen-limited conditions were studied in chemostat cultures. Unexpectedly, the initial response of maltose-grown cultures to oxygen limitation was very similar to that of glucose-grown cultures. In both cases, alcoholic fermentation occurred after a lag phase of 1 h, during which glycerol, pyruvate and D-lactate were the main fermentation products. After ca. 10 h the behaviour of the maltose- and glucose-grown cultures diverged: ethanol disappeared from the maltose-grown cultures, whereas fermentation continued in steady-state, oxygen-limited cultures grown on glucose. The disappearance of alcoholic fermentation in oxygen-limited chemostat cultures growing on maltose was not due to a repression of the synthesis of pyruvate decarboxylase and alcohol dehydrogenase. The results demonstrate that the Kluyver effect for maltose in C. utilis does not reflect an intrinsic inability of this yeast to ferment maltose, but is caused by a regulatory phenomenon that affects a key enzyme in maltose metabolism, probably the maltose carrier. The observed kinetics indicate that this regulation occurs at the level of enzyme synthesis rather than via modification of existing enzyme activity.