Phosphoenolpyruvate carboxykinase (PEPCK) catalyses the reversible reac-tion of decarboxylation and phosphorylation of oxaloacetate (OAA) to gener-ate phosphoenolpyruvate (PEP) and CO2 playing mainly a gluconeogenic rolein green algae. We found two PEPCK isoforms in Chlamydomonas rein-hardtii and we cloned, purified and characterised both enzymes. Chlre-PEPCK1 is more active as decarboxylase than ChlrePEPCK2.ChlrePEPCK1 is hexameric and its activity is affected by citrate, phenylala-nine and malate, while ChlrePEPCK2 is monomeric and it is regulated bycitrate, phenylalanine and glutamine. We postulate that the two PEPCK iso-forms found originate from alternative splicing of the gene or regulated prote-olysis of the enzyme. The presence of these two isoforms would be part of amechanism to finely regulate the biological activity of PEPCKs.