Abstract A comparative study of three enzymes, aconitase, fumarase, and DPN-linked iso citric dehydrogenase was made in normal baker's yeast and in the mutant “petite colonie” grown under varying conditions of oxygen supply. When both normal and mutant yeasts are grown anaerobically, aconitase, fumarase, and DPN- iso citric dehydrogenase activities in the two strains are alike. When grown in the presence of air, all three enzymes show increased activity, but under such conditions distinct differences between the two strains become apparent: the increases in the normal strain are much more pronounced than in the mutant strain. Aeration of non-proliferating suspensions of anaerobically grown normal yeast which is known to re-establish ability to respire also leads to an increased formation of aconitase and, to a lesser extent, fumarase, but in order to obtain their full complement of these enzymes, extensive growth of the cellsi n the presence of air seems necessary. By differential centrifugation all three enzymes were found to be associated mainly with the supernatant fraction in both normal and mutant yeast. The function of these enzymes in the metabolism of yeast is discussed.