  We analyse a supersymmetric string-inspired model of all fermion masses and
mixing angles based on the Pati-Salam $SU(4)\times SU(2)_L \times SU(2)_R$
gauge group supplemented by a $U(1)_X$ flavour symmetry.
  The model involves third family Yukawa unification and predicts the top mass
and the ratio of the vacuum expectation values $\tan \beta$.
  The model also provides a successful description of the CKM matrix and
predicts the masses of the down and strange quarks.
  However our main focus is on the neutrino masses and MNS mixing angles, and
we show how the recent atmospheric neutrino mixing observed by
Super-Kamiokande, and the MSW solution to the solar neutrino problem lead to
important information about the flavour structure of the model near the string
scale.
  We show how single right-handed neutrino dominance may be implemented by the
use of ``Clebsch zeros'', leading to the LMA MSW solution, corresponding to
bi-maximal mixing. The LOW MSW and SMA MSW solutions are also discussed.
