Deformation behavior of polysynthetically-twinned lamellar gamma -TiAl + alpha (2)-Ti3Al single crystals has been analyzed using a three-dimensional, isothermal, rate-dependent, large-strain, crystal-plasticity based materials constitutive model. Within the model it is assumed that plastic deformation parallel to the gamma -TiAl/alpha (2)-Ti3Al lamellar boundaries is controlled by the softer gamma -TiAl phase while deformation which contains a component normal to these boundaries is dominated by the harder alpha (2)-Ti3Al phase. The parameters appearing in the crystal-plasticity materials constitutive relations are assessed using the available experimental information pertaining to the active slip systems, their deformation resistances and hardening and rate behavior of the two constitutive phases both in their single-crystalline and in polysynthetically-twinned lamellar forms. The constitutive relations are implemented in a Vectorized User Material Subroutine (VUMAT) of the commercial finite element program Abaqus/Explicit within which the material state is integrated during loading using an explicit Euler-forward formulation. The results obtained suggest that the adopted crystal-plasticity model and the parameters assessed in the present work account quite well for the observed room-temperature deformation behavior of polysynthetically-twinned lamellar gamma -TiAl + alpha (2)-Ti3Al single crystals.