  We study a supersymmetric version of the seesaw mechanism type-III. The model
consists of the MSSM particle content plus three copies of 24 superfields. The
fermionic part of the SU(2) triplet contained in the 24 is responsible for the
type-III seesaw, which is used to explain the observed neutrino masses and
mixings. Complete copies of 24 are introduced to maintain gauge coupling
unification. These additional states change the beta functions of the gauge
couplings above the seesaw scale. Using mSUGRA boundary conditions we calculate
the resulting supersymmetric mass spectra at the electro-weak scale using full
2-loop renormalization group equations. We show that the resulting spectrum can
be quite different compared to the usual mSUGRA spectrum. We discuss how this
might be used to obtain information on the seesaw scale from mass measurements.
Constraints on the model space due to limits on lepton flavour violating decays
are discussed. The main constraints come from the bounds on the decay mu to e
and gamma but there are also regions where the decay tau to mu and gamma gives
stronger constraints. We also calculate the regions allowed by the dark matter
constraint. For the sake of completeness, we compare our results with those for
the supersymmetric seesaw type-II and, to some extent, with type-I.
