Phosphoenolpyruvate carboxylase of Bryophyllum fedtschenkoi was shown to exist in two forms: a night form, which is phosphorylated and has low sensitivity to inhibition by malate, and a day form, which is dephosphorylated and 10 times more sensitive to malate. The day and night forms of the enzyme were purified retaining their distinct malate sensitivities and phosphorylation states. The purified enzymes contained a major protein (subunit Mr 112,000) and a minor protein (subunit Mr 123,000). The two polypeptides appeared to have closely related amino acid sequences and were present in a similar ratio in extracts that had been prepared rapidly. The phosphate present in the night form of the enzyme was covalently bound to serine. It was not a catalytic intermediate. Alkaline phosphatase removed the phosphate group in vitro and increased the malate sensitivity of the enzyme to that observed for the day form. Both the day and night forms of the enzyme were probably tetramers, and their apparent Mr was lowered by the presence of malate, but was unaffected by Mg2+ ions, EDTA, a rise in pH or a 10-fold change in enzyme concentration. The rapid loss of malate sensitivity, observed in extracts of leaves prepared during the day and at night, was shown to be due to proteolysis of the enzyme. It was slowed in the presence of malate and by phosphorylation of the enzyme.