The low ductility and toughness of β-NiAl alloys near room temperature pose major problems in their potential application as structural materials. The inability of the material to generate and move a sufficient density of dislocations at applied stresses below the fracture stress is the major cause for this inherent brittleness. A directionally solidified β+(γ+γ') composite of nominal composition Ni50Fe30Al20 (at.%) has been used to investigate the effect of interphase boundaries on the mechanical behavior of β phase. The composite exhibits 10% tensile elongation to fracture at room temperature. Observation of slip traces and dislocation substructures shows that the normally brittle β phase undergoes extensive plastic deformation afforded by slip transfer from the plastically soft (γ+γ') phase mixture across the semi-coherent β/(γ+γ') interface. The effect of the orientation relationship between the two phases and the interface strength on the transfer of slip across the interphase boundary is discussed.