The stacked lamellar morphology commonly found in extruded semicrystalline materials has a strong influence on the flow direction, with respect to the loading direction, and on the stability and localization phenomena in tensile experiments. A multiscale numerical model was used to simulate the effect on the macroscopic behavior of a stacked lamellar microstructure. The model established a link between the microscopic, the mesoscopic, and the macroscopic levels. The constitutive properties of the material were identified for the crystallographic and amorphous domains. The average fields of an aggregate of individual phases, having preferential orientations, formed the constitutive behavior of the extruded material. The microscopic morphology of the extruded high-density polyethylene is based on wide-angle X-ray diffraction experiments. The macrostructure was described by a finite element model. The microstructure-induced deformation hardening in the extrusion direction was found to stabilize the macrostructure when it was loaded in the flow direction. (C) 2004 Wiley Periodicals, Inc.