Deletion mutants of transketolase and transaldolase were examined for their growth on xylulose medium. Xylulose can be phosphorylated and degraded via the pentose-phosphate pathway in yeast. Deletion mutants of transaldolase (TAL1) can still grow very slowly with xylulose as the carbon source. In-vitro mutagenesis of TAL1 showed that lysine144 is essential for the catalytic activity of transaldolase. Two genes code for transketolase in S. cerevisiae. Mutants in the gene TKL2 are unaffected by their growth on xylulose medium, while deletion of TKL1 leads to mutants with only very slow growth when xylulose is used as the carbon source. Double mutants for TKL1 and TKL2 could not use xylulose as the carbon source at all, indicating a complete block of the transketolase reaction. Transformants overexpressing transaldolase, transketolase or both enzymes were constructed by placing their genes on the multicopy vector GS42. The activity of transaldolase or transkatolase, or a combination of both enzymes, could be increased by more than 100 fold compared to the wild-type strain measured in parallel. Growth of the transformants on agar plates with xylulose as the carbon source was not accelerated compared to the wild-type strain.