The utilization of glucose by Scenedesmus quadricauda has been studied by manometric methods. With small amounts the oxidation only goes to about 16 per cent. of completion, the remainder of the glucose being built up into an acid-hydrolysable polysaccharide which is not starch, and which is built up from a hexose other than fructose. The extent of the oxidation is not affected by sodium azide or 2:4-dinitro-phenol, but can be varied by altering the pH of the suspension medium; more oxygen is absorbed per unit weight of glucose at pH 8 than at pH 5. It is suggested that there is no fixed relation between the two processes of oxidation and synthesis, as has previously been suggested, but that the glucose, after phosphorylation, saturates the oxidation system, and the excess hexose-phosphate is built up into a polysaccharide. Mannose is also utilized in the same way, but at a slower rate, and less oxygen is used up compared with glucose. Galactose gives only a slight stimulation of the respiratory rate, and fructose, sucrose, and lactose are not absorbed.