  In practically all extentions of the standard model, the neutrinos naturally
acquire a mass. The neutrino mass matrix, however, contains many parameters
which can neither be predicted by the prevailing models nor can be fitted to
the data. We propose a fermion mass matrix ansatz in the context of Grand
Unified Supersymmetric Theories (GUTs) at the GUT scale and use the
renormalization group equations for the gauge and Yukawa couplings to predict
the 13 arbitrary parameters of the Standard model. The constraints imposed by
the charged fermion data are used to make predictions in the neutrino sector.
The neutrino mass matrix is worked out in the case of the flipped SU(5) model
and the predictions are compared with the available experimental data.
