The inhibitory effect of shikimic acid on phosphoenolpyruvate carboxylase (PEPC) from Amaranthus leaves was evaluated. Shikimic acid behaved as a competitive inhibitor vs. phosphoenolpyruvate (PEP). Ki for PEPC inhibition was 22 μM. Phenylalanine, tyrosine and tryptophan did not show any effect on PEPC activity.