The glucose analog, 3-O-methyl-d-glucose, inhibited growth of yeast on non-fermentable carbon sources. The sugar was phosphorylated by the yeast and also in vitro, by a commercial preparation of yeast hexokinase. The chromatographic behaviour of the phosphorylated product was identical in both cases. This suggests that 3-O-methyl-d-glucose is phosphorylated to form 3-O-methyl-d-glucose 6-phosphate. The inhibition of the growth appears to be due to interference with the derepression of several enzymes necessary to grow on non-fermentable carbon sources. Spontaneous mutants whose growth was unaffected by 3-O-methyl-d-glucose were isolated. In these mutants there was no significant accumulation of the phosphorylated ester and the derepression of the enzymes tested was not affected by the glucose analog.