The deformation behavior of individual Widmanstatten colonies comprised of aligned lamellae of ductile phases has been investigated. Based on the α β Ti alloy, Ti-8Al-lMo-lV, this study shows the existence of a large (>2X) variation in the critical resolved shear stress for yielding of individual colonies. Schmid’s Law is not obeyed except for prism slip parallel to the β lamellae. In addition, colonies with a high yield stress exhibit a high work hardening rate and fine, uniform slip. This behavior appears to be independent of the α-phase slip system (basal, prism, or pyramidal) and of the microstructure (α βvs α α′ (martensite)). The experimental behavior is correlated to several colony orientation parameters including the stress axis, the slip plane, the slip direction and the orientation of the α β interface. The yield stress of a colony is found to increase as the slip direction of the dominant macroscopic slip plane approaches normality to the α β interface. These results indicate that the macroscopic flow behavior of colonies comprised of ductile lamellae depends on the ability of a slip system, once activated in the softer phase, to shear through the harder phase. The data also indicate that the interaction stresses at the phase interfaces are not a principal factor controlling macroscopic yielding in α β Ti alloys. Finally, the alignment of a slip system in the α-phase with a potential slip system in the β-phase lamellae does not appear to affect the yield stress strongly.