Abstract: A method of forming an extrusion die comprises forming an extrusion aperture in a die piece, forcing a flowable material partly through the extrusion aperture so that the leading surface of the material becomes deformed in the extrusion direction, and then removing the exposed surface of the extrusion aperture, by chemical or electro-etching, so that the effective axial bearing depth of the extrusion aperture at any location around its periphery corresponds to the deformation of the leading surface of the flowable material at that location. The axial bearing depth of the completed die at any location is thus inversely proportional to the resistance to flow at that location, resulting in substantially uniform flow over the whole of the die aperture.