Ultrasonic irradiation during solidification has emerged as an effective means of structural refinement for both metallic and non-metallic materials. A number of mechanisms have been proposed to explain the origin of such refinement in an attempt to both understand and exploit the refining potential. One such mechanism proposed is based on the hypothesis of formation and vibration-stimulated separation of wall crystals. This work examines the role of wall crystals in ultrasonic refinement of pure magnesium and Mg-3Al-1Zn alloy, where a titanium sonotrode is immersed into the melt for direct ultrasonication at an amplitude of 30 mu m and a typical frequency of 20 kHz. Evidence is produced from different perspectives to show that the wall crystal effect has a negligible influence on the significant refinement observed under ultrasonic exposure. Instead, all of the experimental evidence produced consistently supports the hypothesis of microstructural refinement originating from cavitation-enhanced heterogeneous nucleation. (C) 2009 Elsevier B.V. All rights reserved.