Over the last thirty years it has become clear that protein phosphorylation/dephosphorylation plays a central role in the regulation of many processes and pathways in both eukaryotic and prokaryotic cells (e.g. Edelman et al., 1987; Cohen, 1988). Relatively little attention has been paid to protein phosphorylation in higher plants, but several important targets of phosphorylation have now been identified (see e.g. Budde & Chollet, 1988, Ranjeva & Boudet, 1987 for reviews). Among the plant enzymes that have been shown to be regulated by phosphorylation are pyruvate dehydrogenase (e.g. Randall et al., 1981), pyruvate, phosphate dikinase (Burnell & Hatch, 1985), quinate:NAD+ oxidoreductase (Refeno et al., 1982), sucrose phosphate synthase (Huber et al., 1989) and phosphoenolpyruvate carboxylase (PEPc) (see below). In this article we focus on the control of PEPc by phosphorylation in leaf tissue of C4 and Crassulacean acid metabolism (CAM) plants.