  We analyze the one-loop effects (strong and electroweak) on the
unconventional top quark decay mode $t\rightarrow H^{+} b$ within the MSSM. The
results are presented in the on-shell renormalization scheme with a physically
well motivated definition of $\tan\beta$. The study of this process at the
quantum level is useful to unravel the potential supersymmetric nature of the
charged Higgs emerging from that decay. As compared with the standard mode
$t\rightarrow W^{+} b$, the corrections to $t\rightarrow H^{+} b$ are large,
slowly decoupling and persist at a sizeable level even for all sparticle masses
well above the LEP 200 discovery range. As a matter of fact, the potential size
of the SUSY effects, which amount to corrections of several ten percent, could
counterbalance the standard QCD corrections and even make them to appear with
the ``wrong'' sign. Therefore, if the charged Higgs decay of the top quark is
kinematically allowed -a possibility which is not excluded by the recent
measurements of the branching ratio $BR(t\rightarrow W^{+} b)$ at the Tevatron
- it could be an invaluable laboratory to search for ``virtual'' supersymmetry.
While a first significant test of these effects could possibly be performed at
the upgraded Tevatron, a more precise verification would most likely be carried
out in future experiments at the LHC.
