  Within the framework of the MSSM, we compute the electroweak one-loop
supersymmetric quantum corrections to the width $\Gamma (t\rightarrow W^{+}\,
b)$ of the canonical main decay of the top quark. The results are presented in
two on-shell renormalization schemes parametrized either by $\alpha$ or $G_F$.
While in the standard model, and in the Higgs sector of the MSSM, the
electroweak radiative corrections in the $G_F$-scheme are rather insensitive to
the top quark mass and are of order of $1\%$ at most, the rest (``genuine''
part) of the supersymmetric quantum effects in the MSSM amount to a
non-negligible correction that could be about one order of magnitude larger,
depending on the top quark mass and of the region of the supersymmetric
parameter space. These new electroweak effects, therefore, could be of the same
order (and go in the same direction) as the conventional leading QCD
corrections.
