The electrical properties of the outer layer of the cell wall of a hydrocarbon-grown yeast C. guilliermondii were studied in detail by cell electrophoresis. Some experiments were also made with the yeast S. cerevisiae. The mobilities of the yeasts were measured as a function of pH at pH 2-11.5 and constant ionic strength I = 0.02 mol/l. For identification of surface groups the pH-mobility curves are used to calculate pK-values. The results indicate the presence of amino and carboxyl groups on the surface of C. guilliermondii. These groups are probably a part of a glyco-protein with an isoelectric point at pH 3.3 located on the surface. Electrophoresis of droplets of mineral oil coated with adsorbed protein layer confirms these conclusions. The importance of this glycoprotein in hydrocarbon uptake is discussed.