The anaplerotic and gluconeogenetic metabolism of baker's yeast was studied at the enzymatic level during glucose-ethanol diauxic growth in the presence and absence of aspartate. Of the two possible anaplerotic systems, only the pyruvate carboxylase by-pass was present during the whole growth process. The second system, the glyoxylate by-pass (isocitrate lyase as the indicator), like the specific enzymes of the gluconeogenetic metabolism, phosphoenolpyruvate carboxykinase and hexosediphosphatase began to appear only after the glucose had been consumed. The addition of glucose during the growth phase based on ethanol effected a rapid disappearance of phosphoenolpyruvate carboxykinase and hexosediphosphatase activities. The activity of pyruvate carboxylase decreased when the growth medium was supplied with asparate. The presence of aspartate had no effect on the activities of the other enzymes studied.