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Nutrient sensors allow cells to adapt their metabolisms to match nutrient availability by regulating metabolic pathway expression. Many such sensors are cytosolic receptors that measure intracellular nutrient concentrations. One might expect that inducing the metabolic pathway that degrades a nutrient would reduce intracellular nutrient levels, destabilizing induction. However, in the galactose-responsive (GAL) pathway of Saccharomyces cerevisiae, we find that induction is stabilized by flux sensing. Previously proposed mechanisms for flux sensing postulate the existence of metabolites whose concentrations correlate with flux. The GAL pathway flux sensor uses a different principle: the galactokinase Gal1p both performs the first step in GAL metabolism and reports on flux by signalling to the GAL repressor, Gal80p. Both Gal1p catalysis and Gal1p signalling depend on the concentration of the Gal1p-GAL complex and are therefore directly correlated. Given the simplicity of this mechanism, flux sensing is probably a general feature throughout metabolic regulation.