Nine of eleven yeasts of different species or genera grew in the presence of air on the intermediate of D-xylose catabolism, D-xylulose (D-threo-pentulose). Growth on this substrate was efficient as judged by the optical density in stationary phase being generally similar to that after growth on glucose. Yeasts which grew on D-xylose also did so on D-xylulose, but among those which grew are included several which utilise neither D-xylose nor xylitol: Saccharomyces cerevisiae, Saccharomyces carlsbergensis, and Schizosaccharomyces pombe. Since catabolism of a sugar generally requires an initial phosphorylation step, growth of these strains suggests that they contain an enzyme which can function as a D-xylulose kinase. The D-xylulose-5-phosphate formed thereby is considered to enter the pentose-phosphate pathway. Glucose-grown inocula of S. carlsbergensis and Schizosaccharomyces pombe, and of several other yeasts, began to grow logarithmically when placed on D-xylulose with no apparent delay, or one which was minimal, suggesting that the D-xylulose kinase was already present in such cells, or was rapidly induced. Petites of S. cerevisiae did not grow on D-xylulose indicating that, in this species, mitochondria are involved in its utilisation.