Abstract: A single screw extruder comprises a mixer associated with the screw, and comprising a rotor driven by the screw and a stator, the rotor and the stator each carrying mutually facing interengaging rings of teeth whereby the material is urged outwardly from the screw along a first tortuous mixing path, and then returned inwardly to the screw along a second tortuous mixing path, with the teeth extending axially, or generally so, with respect to the longitudinal axis of the screw. The invention also includes a mixer comprising a cylindrical stator chamber having opposed, radially extending faces provided with axially projecting, radially spaced-apart rings made up of alternating teeth and ridges, a rotor rotatably fitted within the stator and provided on its opposite side faces with axially projecting, radially spaced-apart rings of alternating teeth and ridges with the rotor and stator rings interengaging with both radial and axial clearance so as to define a tortuous material flow path.