  We study the supersymmetric spontaneous symmetry breaking of SO(10) into
SU(3)xSU(2)xU(1) for the most physically interesting cases of SU(5) or flipped
SU(5)xU(1) intermediate symmetries. The first case is more easily realized
while the second one requires a fine-tuning condition on the parameters of the
superpotential. This is because in the case of SU(5) symmetry there is at most
one singlet of the residual symmetry in each SO(10) irreducible representation.
We also point out on more general grounds in supersymmetric GUT's that some
intermediate symmetries can be exactly realized and others can only be
approximated by fine-tuning. In the first category, there could occur some
tunneling between the vacua with exact and approximate intermediate symmetry.
The flipped SU(5)xU(1) symmetry improves the unification of gauge couplings if
(B-L) is broken by (B-L)=1 scalars yielding right handed neutrino masses below
10^{14} GeV}.
