The crack growth across a boundary between two colonies, i.e. regions of differing lamellar orientation, in two-phase lamellar Ti-Al is studied computationally to quantify the influence of such boundaries on toughening, as observed in recent in-situ fracture studies. The model represents the lamellar Ti-Al as gamma-phase lamellae, modeled as bulk elastic-viscoplastic material, interspersed with alpha(2)-phase lamellae for which either the alpha(2) phase or alpha(2)-gamma interface are considered as weak planes for fracture. Computationally, dynamic plane-strain analyses of the crack propagation are carried out. Fracture in both phases is accommodated using a cohesive surface formulation that permits crack growth and nucleation to evolve naturally. Results show that the lamellar misorientation across a boundary, the thickness of the boundary region, and the spatial offset between successive weak lamellae, all play a role in inhibiting crack propagation across the boundary. The gamma phase plasticity has a comparatively small influence on the toughening. The enhancements in applied stress intensities required to nucleate cracks across the colony boundary are comparable to those observed experimentally when the weak-plane spacing is comparable to the spacing of microcracks and crack plane offsets observed experimentally. (C) 2002 Elsevier Science B.V. All rights reserved.