Phosphofructokinase is identified as the regulator reaction activated at the onset of the climacteric rise in respiration of the ripening tomato fruit (Lycopersicon esculentum Mill). The concentration of ATP in the fruit increases to a maximum value after the climacteric peak of respiration is past. Orthophosphate is proposed as the most probable activator of phosphofructokinase in the ripening fruit.Fifteen hours after infiltrating tomato fruit with orthophosphate, the rate of respiration increased and remained high until the end of the experiment, 45 hours after infiltration. In experiments where tomato plants were grown at various nutrient levels of P, the rate of respiration when fruit harvested at the mature-green stage reached the respiratory climacteric was correlated with the concentration of orthophosphate in the fruit at the end of the experiment. These results are consistent with the hypothesis that stimulation of phosphofructokinase through increasing concentration of orthophosphate in the cytoplasm of the fruit contributes to the climacteric rise in respiration.