Single-colony thick compact tension specimens of binary lamellar Ti-46.5%Al were tested within a scanning electron microscope to examine the contribution of colony boundaries to crack growth resistance under monotonic loads. These specimens were obtained by machining slices from bulk material that had been heat treated to grow the colony size. Thus, the lamellae in adjacent colonies exhibit significant misorientation across the boundary. The orientation of the lamellae within a colony has been characterized in terms of two angles defined with respect to the notch orientation: an in-plane angle β and a through thickness angle β. The change in these two angles across the colony boundary quantifies the misorientation. In addition a third angle, ö, defines the colony boundary tilt to the vertical plane. These parameters were measured in several specimens and the crack growth resistance across the boundary was qualitatively and quantitatively characterized. The importance of the through-thickness angle β in providing resistance to crack growth is illustrated.