The synthesis of organic acids was studied during fruit development of two peach (Prunus persica L, Batsch) cultivars, Fantasia and Jalousia, having fruits with high and low organic acid content, respectively. The malate content was higher in cv. Fantasia than in cv, Jalousia at the end of the first rapid growth stage (50 days after bloom [DAB]). Malate and citrate contents,were higher in Fantasia than in Jalousia during the second rapid growth stage (from 100 DAB to maturity). The expression of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31), which is involved in organic acid synthesis, was studied during peach fruit development. PEPC mRNA levels, and protein levels on a total soluble protein basis, peaked at 23 and 108 DAB in Fantasia. In Jalousia, they were very low at 23 DAB and reached levels similar to Fantasia at 108 DAB, For both cultivars, in vitro PEPC activity expressed on a dry weight basis was maximal at 24 DAB, decreased from 24 to 60 DAB, and then remained constant. The activity of peach fruit PEPC appeared extremely sensitive to malate (I-0.5 of 100 mu M for Fantasia and 65 mu M for Jalousia at pH 7.3) and low pH, PEPC may participate in the control of organic acid accumulation during fruit development in the normal-acid fruit of Fantasia, However, mechanisms other than organic acid synthesis might account for the differences in acidity between normal-acid and non-acid peach fruit.