Abstract.Purified and crude phosphoenolpyruvate carboxylase from the CAM plantKalanchoë daigremontianaHamet et Perrier (Bryophyllum diagremontianum) was assayed at temperatures between 10 and 45° C. The optimum temperature of the enzyme activity changed with substrate availability and effector concentration in the assay.l‐malate inhibited the enzyme activity and lowered the optimum temperature. Glucose‐6‐phosphate raised the optimum temperature to 43°C.Kmvalues for phosphoenolpyruvate increased with assay temperature from 0.12 mol m‐3at 15° C to 0.36 molm−3at 35° C. Inhibition by malate increased with temperature and acidity of the assay. In the crude enzyme 50% of control activity was inhibited by 1.65 mol m‐3malate at 15° C and by 0.5 mol m‐3at 35° C (at pH 7.0). With purification malate sensitivity was lost (Kivalues for malate at least 10 times higher). The shift in optimum temperatures for PEP‐carboxylase activity thus results from changes in the kinetic parameters with temperature and allosteric effectors. The often low optimum temperatures for CO2fixation observed in nature may thus be the result of substrate and effector concentrations in the cytoplasm and the antagonistic effect of temperature on substrate affinity and effector efficiency on phosphoen