Ultrasonic diagnostic method of screw behavior during extrusion has been designed, developed, and carried out on a 63.5-mm single screw extruder, equipped with either a basic conveying screw or a mixing screw. It was found that the screw behavior depends on the screw speed and the die pressure. Whereas initially at low speed, the screw centered by the hydrodynamic forces rotates smoothly in the axial position, at higher screw speeds and high die pressures the screw starts vibrating. Thus, (1) the screw tip vibrated with the amplitude of 10-210 mu m as the screw speed varied from 5 to 100 revolutions per minute; (2) the screw vibrations in the vertical and horizontal directions were asymmetrical; (3) the vibration pattern repeated itself at every two revolutions; (4) the vibrations originated in the poorly aligned engagement in the gearbox sleeve when the screw was pushed deeply into it by backpressure; (5) once pushed in the screw oscillated at any screw speed or die pressure. POLYM. ENG. SCI., 49:244-250, 2009. Published by the Society of Plastics Engineers