We have recently described the isolation of a mitochondrial rotenone-insensitive NADH: ubiquinone oxidoreductase from Saccharomyces cerevisiae [de Vries, S. & Grivell, L. A. (1988) Eur. J. Biochem. 176, 377–384]. We now report the isolation of the nuclear gene encoding this single-subunit enzyme. Null mutants have been constructed by means of one-step gene disruption. Oxygen-uptake experiments, performed with mitochondria isolated from the mutant cells, showed that this NADH dehydrogenase catalyzes the oxidation of NADH generated inside the mitochondrion. Inactivation of this NADH dehydrogenase does not affect growth on glucose and ethanol, but growth on lactate, pyruvate and acetate is impaired or absent. This phenotype is discussed in terms of the interplay between different metabolic pathways in yeast.