The affinity constant Ka of PPi-PFK for Fru 2,6-P2 is equal to 1.56 nM for the potato enzyme and to 6.67 nM for that of the mung bean in the absence of chloride ions. These results are notably lower than the currently reported 5.5 nM and 30, 50 nM respectively. It is shown that the chloride ion is a competitive inhibitor of Fru 2,6-P2 for both enzymes. The inhibition constant Ki is equal to 15.6 mM for potato PPi-PFK up to 40 mM chloride. For the mung bean enzyme, the Ki is 19.0 mM up to 30 mM chloride. No effects are detected on the Michaelis­Menten constants Km of the substrates Fru-6-P and PPi up to 40 mM chloride. Other halide ions are also found to inhibit the potato PPi-PFK: bromide is competitive like chloride, whereas fluoride and iodide have a mixed inhibition towards Fru 2,6-