An extrusion blow mounding control system for controlling parison wall thickness in an extrusion blow moulding machine comprises a wall thickness controller 10 for controlling the width of an extrusion die gap 5 defining parison wall thickness within the machine, an interface unit 14 for supplying an input signal to the wall thickness controller 10 dependent on an actual position signal from a position transducer 12 indicative of the actual position of a movable die member 6 delimiting the die gap 5, and an output for supplying a desired position signal from the wall thickness controller to an electrohydraulic servo valve 9 for controlling the position of the movable die member 6 to provide a desired width of the die gap 5. The interface unit 14 comprises signal processing circuitry for receiving the actual position signal from the position transducer 12 and a feedback signal from an automatic weighing system 16 indicative of weight variation of product outputted by the machine and for producing the input signal for the wall thickness controller 10 in dependence on the actual position signal and the feedback signal. Such a closed loop control system can be used to control the weight of the final product within a narrow weight range, and this results in improved product consistency and quality, as well as enabling the total quantity of moulding material used in moulding of the product to be decreased.