We construct and analyze an SU(6) X SU(2) GUT. The model is k=1 string
embedable in the sense that we employ only chiral representations all
owed at the k = 1 level of the associated Kac-Moody Algebra. Both case
s SU(6) X SU(2), and SU(6) X SU(2), are realized. The model is charact
erized by the SU(6) X SU(2) --> SU(4) X SU(2) X SU(2) breaking scale M
-x, and the SU(4) X SU(2) X SU(2) --> SU(3)(C) X SU(2)(L) X U(1)(Y) br
eaking scale M-R. The spectrum bellow M-R includes an extra pair of ch
arge-1/3 colour-triplets of mass M-I less than or equal to M-R that do
es not couple to matter fields and, possibly, an extra pair of isodoub
lets. Above M-X the SU(6) and SU(2) gauge couplings always unify at a
scale which can be taken to be the string unification scale M-s simila
r to 5 X 10(17) GeV. The model has Yukawa coupling unification since q
uarks and leptons obtain their masses from a single Yukawa coupling. N
eutrinos obtain acceptably small masses through a see-saw mechanism. C
oloured triplets that couple to matter fields are naturally split from
the coexisting isodoublets without the need of any numerical fine tun
ing. (C) 1997 Elsevier Science B.V.