A constitutive model for dynamic strain ageing together with a three-dimensional finite element analysis were used to simulate discontinuous yielding behaviour associated with the Portevin-Le Chatelier effect. The deformation behaviour of both fiat and round specimens of a dynamically strain ageing material was simulated, utilizing constitutive parameters for an Al-Mg-Si alloy. Finite element simulations showed the occurrence of propagative zones of localized deformation and provided insight into the strain rate distribution in the zones, characterized by a sharp strain rate peak. The localized deformation zones in round specimens were found to have a diffuse double cone morphology. Propagative deformation bands in flat strip specimens were planar and inclined with respect to the tensile axis by approximately 35 degrees. The dependence of the morphology of the localized deformation zones on specimen geometry and deformation conditions, as well as the serrated stress-time curves obtained, are in good agreement with experimental measurements in Al-Mg-Si. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.