Abstract: A pipe of thermoplastic polymer, e.g. polyvinylchloride is extruded by an extruder comprising an extrusion die (2) having a moulding end part (2a) with heating elements (11) located about said end part. The wall thickness of the pipe is controlled in order to obtain a pipe of a substantially even wall thickness. For said control the wall thickness of the extruded pipe is measured at 360 points equally distributed over the circumference and moreover the average is determined in eight measuring sectors comprising 45 measuring points per sector. The average of the measuring values of one measuring sector is compared with the circumferential average of the 360 points, the deviation of the desired wall thickness in said one sector being counter balanced by the action of the heating elements. In a preferred embodiment a polyvinylchloride pipe obtained by means of a two screw extruder presents an even wall thickness over the circumference.