A mutant of the C4 plant, Amaranthus edulis, which lacks the C4 form of phosphoenolpyruvate carboxylase (PEPC), has been generated In Lancaster. This has been used to generate heterozygous plants with a range of activities of PEPC which have been employed to analyse the degree of control over photosynthetlc CO2 assimilation that Is exercised by PEPC. This is the first time that control analysis has been done In a C4 plant. Heterozygous plants showed no change In the contents of soluble protein, chlorophyll, or NAD-mallc enzyme. A reduction In PEPC activity led to a decrease In the carboxylatlon efficiency and to larger decreases In the rate of photosynthetlc C02 fixation at lower C02 concentrations and at higher light Intensities. Examples of control coefficients (CPEPC) ranged from 0.26 in Wt in air (Ci 130) to 0.54 in low CO2 (Cl 30) to 1.00 In 45* PEPC in low CO2> I.e. unusually high In the wild-type for a highly regulated enzyme such as PEPC. There was also some evidence that below 5O5K PEPC there may be some compensating mechanism operating to reduce the effect of the loss in PEPC protein.