Deformation behavior of fully-lamellar polycrystalline gamma-TiAl + alpha(2)-Ti3Al alloys has been analyzed using a finite element method. A three-dimensional rate-dependent, finite-strain, crystal-plasticity based materials constitutive model is used to represent the deformation behavior of the bulk material. The constitutive behavior of gamma-TiAl/gamma-TiAl lamellar interfaces and lamellae-colony boundaries, on the other hand, are modeled using a cohesive-zone formulation. The interface/boundary potentials used in this formulation are determined through the use of atomistic simulations of the interface/boundary decohesion. The constitutive relations for both the gamma-TiAl + alpha(2)-Ti3Al bulk material and the lamellar interfaces and colony boundaries are implemented in the commercial finite element program Abaqus/Standard within which the material state is integrated using an Euler-backward implicit formulation. The results obtained show that plastic flow localizes into deformation bands even at an overall strain level of only 0.5% and that incompatibilities in plastic flow between the adjacent colonies can give rise to high levels of the hydrostatic stress and, in turn, to intercolony fracture. Furthermore, it is found that when lamellar interfaces are admitted into colonies, fracture is delayed and the materials fail in a more gradual manner. (C) 2003 Kluwer Academic Publishers.