AbstractThe glycolytic pathway in Saccharomyces cerevisiae is activated by fermentable sugars at several steps. Mutants with deletions of genes coding for enzymes of the upper part of glycolysis were used to characterize the triggering mechanisms. Synthesis of fructose‐2,6‐bisphophate is catalysed by two 6‐phosphofructo‐2‐kinase isoenzymes, one of which is activated by fermentable sugars while synthesis of the second enzyme is induced (Kretschmer and Fraenkel, 1991). Increase in the level of fructose‐2, 6‐bisphosphate is demonstrated to depend on an internal metabolite upstream of the phosphoglucose isomerase reaction. The signalling process correlates with distinct temporal changes in the concentration of glucose‐6‐phosphate but not with its absolute level, indicating an adaptational mechanism. It is independent of the uptake and phosphorylation systems used by different sugars. Interestingly, this increase, although delayed, could also be observed in strains lacking the rapid cAMP increase after sugar addition which is thought to be responsible for the activating process. Synthesis of glucose‐6‐P and fructose‐6‐P is needed for the complete induction of pyruvate kinase and inactivation of fructose‐1,6‐bisphosphatase. On the other hand, induction of pyruvate decarboxylase depends mainly on a signal in the lower part of glycolysis.