The flow behavior of two molten linear high-density polyethylenes (HDPE) is carefully studied using a single screw extruder equipped with a transparent slit die. Flow curves are drawn for both resins and compared. Slit die experiments are performed using birefringence technique to obtain stress fields, and image analysis is developed to visualize the stress transients during the instability. At low flow rate, the extrusion is stable. At more important flow rate, the so-called spurt or stick-slip instability appears. During the instability, periodic birefringence fringes pulsations are observed, synchronized with pressure oscillations. suggesting a periodic stick-slip transition mechanism. Correlation with velocity oscillations is discussed. (C) 2003 Elsevier Science B.V. All rights reserved.