  In the supersymmetric seesaw model of neutrino masses, augmented by the
non-Abelian discrete tetrahedral symmetry A_4, a specific pattern of neutrino
mixing is automatically generated if one of the three heavy singlet neutrino
superfields acquires a nonzero vacuum expectation value. This pattern turns out
to be exactly that of tribimaximal mixing, i.e. sin^2 theta_23 = 1/2, sin^2
theta_12 = 1/3, and sin^2 theta_13 = 0, in good agreement with data.
