The kinetics of L-glucose transport by Rhodotorula glutinis were studied over a 720-fold range of sugar concentrations. Analysis of the saturation isotherm revealed the presence of a one-carrier system for L-glucose in the plasma membrane of Rhodotorula glutinis. This carrier exhibited a km of 3.7 +/- 0.3 mM. D-Ribose was found to be a competitive inhibitor with a Ki of 19 +/- 1 mM. The results suggest that L-glucose is transported by the high-Km, D-ribose carrier. L-Glucose was transported against a concentration gradient and the transport was inhibited by the proton conductor 2,4-dinitrophenol.