  We consider the renormalisation of the Fayet-Iliopoulos D-term in a
softly-broken supersymmetric gauge theory with a non-simple gauge group
containing an abelian factor, and present the associated beta-function through
three loops. We also include in an appendix the result for several abelian
factors. We specialise to the case of the minimal supersymmetric standard model
(MSSM), and investigate the behaviour of the Fayet-Iliopoulos coupling for
various boundary conditions at the unification scale. We focus particularly on
the case of non-standard soft supersymmetry breaking couplings, for which the
Fayet-Iliopoulos coupling evolves significantly between the unification scale
and the weak scale.
