AbstractDay — and night‐forms of phosphoenolpyruvate carboxylase (EC 4.1.1.31) (PEP‐C) were extracted from leaves of the CAM plantKalanchoë daigremontiana. During storage after extraction, the day‐form spontaneously lost its sensitivity towards the allosteric inhibitorl‐malate. This effect was accelerated by inorganic phosphate, PEP, 3‐PGA and G‐6‐P.l‐Malate however stabilized the malate sensitivity of the day‐form of PEP‐C.Crude desalted extracts of the day‐form and of the night‐form of PEP‐C were subjected to ultracentrifugation on a continuous sucrose gradient in the presence of malate and of marker enzymes. Both forms of PEP‐C were found to have relative molecular masses of about 200,000. This suggests that both forms represent the dimers of the enzyme protein. The result also suggests that reversible dissociation and association of enzyme subunits is not the mechanism which brings about the interconversion of the two PEP‐C forms dur