A nozzle ring of an extruder for blowing tubular plastics film is divided into correcting sectors provided with adjusting elements. The thicknesses of the blown film are measured over the circumference thereof and a number of film sectors of equal cross-sectional area corresponding to the number of correcting sectors is established from the measured thicknesses. Assuming that the film sector of maximum or minimum thickness has been extruded in the correct position for the purpose of determining the correcting sector associated with the film sector, the subsequent film sectors of equal cross-sectional area are in turn allocated to successive correcting sectors. The throughput and cross-sectional area or temperature of the respective correcting sector is adjusted until the circumferential lengths of the film sectors are equal.