Large Angle MSW Solution in Grand Unified Theories with SU(3) X U(1) Horizontal Symmetry

We construct a model with a SU(3) X U(1) horizontal symmetry in the context of Grand Unified Theories. In our models, the bi-maximal lepton mixing and suitable neutrino masses for the large angle MSW solution are obtained without any fine-tuning due to the spontaneously broken SU(3)_H symmetry. The three generations of quarks and leptons are unified as members of the SU(3)_H fundamental representation, and the U(1)_H charge gives the origin of the fermion mass hierarchy and mixing angles. We present two explicit examples of SU(5)_GUT and SO(10)_GUT models, in which the Yukawa structures are given successfully.