The uptake of14C-labeled glucose, fructose, and sucrose intoNitella flexilishas been investigated by varying concentrations of sugars, by the use of competition experiments, by the use of inhibitors, and by varying conditions of temperature. The kinetic studies and the complex interaction between the sugars were characteristic of a carrier-mediated system maintained by metabolic energy. Glucose and fructose appear to have different sites of entry though each can influence the other. It is suggested that sucrose is hydrolyzed at the plasmalemma, glucose and fructose then entering the cells.