Yeasts do not form a homogeneous group as far as energy-yielding metabolism is concerned and the fate of pyruvate, a glycolytic intermediate, determines the type of energy metabolism. Kluyveromyces lactis has become an alternative to the traditional yeast Saccharomyces cerevisiae owing to its industrial applications as well as to studies on mitochondrial respiration. In this review we summarize the current knowledge about the K. lactis respirofermentative metabolism, taking into account the respiratory capacity of this yeast and the molecular mechanisms controlling its regulation, giving an up-to-date picture. (C) 2000 Elsevier Science Inc. All rights reserved.