  We analyze the contribution of the SUSY particles to the coupling of the
lightest Higgs boson to two photons in supersymmetric theories. We discuss to
what extent these contributions can be large enough to allow for a
discrimination between the lightest SUSY and the standard Higgs particles in
the decoupling limit where all other Higgs bosons are very heavy and no
supersymmetric particle has been discovered at future colliders. We find that
only chargino and top squark loops can generate a sizeable difference between
the standard and the SUSY Higgs-photon couplings. For masses above 250 GeV, the
effect of chargino loops on the two-photon width is however smaller than $\sim
10%$ in the entire SUSY parameter space. Top squarks heavier than 250 GeV can
induce deviations larger than 10% only if their couplings to the Higgs boson
are large. Since top squark contributions can be sizeable, we derive the
two-loop QCD correction to squark loops and show that they are well under
control.
