The cell volume fraction ofSaccharomyces cerevisiae accessible tod-xylose and other nonmetabolized sugars was found to correspond to 100% of cell water in resting yeast over a range of osmolarities of the medium. This sugar space was decreased to 60% by nitrogen deficiency and to 82% by phosphorus deficiency. The space of N-deficient cells was significantly raised by pre-incubation with ammonium chloride, amino-acids and nucleotides from the yeast extract and by adenosine triphosphate while that of P-deficient cells was not significantly altered by analogous treatment. The xylose space was found to be markedly lower in growing than in resting cells. Urea (0.3–6%) had a depressing effect on the sugar space size while a variety of metabolic inhibitors were without influence. TheKm andV values ofd-xylose uptake bySaccharomyces cerevisiae R XII at 30° were found to be 180–192 mM and 680–730 mg. xylose/ml. cell volume/hr., respectively. These values were not affected by N-deficiency and by enrichment with adenosine triphosphate. The xylose space size was shown to be practically identical with that of urea space and its decrease was related to the increase of “bound” water within the cell under various conditions studied. The changes of space are tentatively attributed to changes of hydration and dehydration of cellular components.