Phosphoenolpyruvate carboxylase (PEPCase, EC 126.96.36.199) is a key enzyme of C(4) photosynthesis. It has evolved from ancestral non-photosynthetic (C(3)) isoforms and thereby changed its kinetic and regulatory properties. We are interested in understanding the molecular changes, as the C(4) PEPCases were adapted to their new function in C(4) photosynthesis and have therefore analysed the PEPCase genes of various Alternanthera species. We isolated PEPCase cDNAs from the C(4) plant Alternanthera pungens H.B.K., the C(3)/C(4) intermediate plant A. tenella Colla, and the C(3) plant A. sessilis (L.) R.Br. and investigated the kinetic properties of the corresponding recombinant PEPCase proteins and their phylogenetic relationships. The three PEPCases are most likely derived from orthologous gene classes named ppcA. The affinity constant for the substrate phosphoenolpyruvate (K (0.5) PEP) and the degree of activation by glucose-6-phosphate classified the enzyme from A. pungens (C(4)) as a C(4) PEPCase isoform. In contrast, both the PEPCases from A. sessilis (C(3)) and A. tenella (C(3)/C(4)) were found to be typical C(3) PEPCase isozymes. The C(4) characteristics of the PEPCase of A. pungens were accompanied by the presence of the C(4)-invariant serine residue at position 775 reinforcing that a serine at this position is essential for being a C(4) PEPCase (Svensson et al. 2003). Genomic Southern blot experiments and sequence analysis of the 3' untranslated regions of these genes indicated the existence of PEPCase multigene family in all three plants which can be grouped into three classes named ppcA, ppcB and ppcC.