  We compute the one-loop supersymmetric QCD quantum effects on the width
$\Gamma (t\rightarrow W^{+}\, b)$ of the canonical main decay of the top quark
within the framework of the MSSM. The corrections can be of either sign
depending on whether the stop squark mass is above or below the top quark decay
threshold into stop and gluino $\Gamma (t\rightarrow\tilde{t} \,\tilde{g})$.
For $m_{\tilde{t}}$ above that threshold, the corrections are negative and can
be of the same order (and go in the same direction) as the ordinary QCD
corrections, even for stop and gluino masses of ${\cal O}(100)\,GeV$. Since the
electroweak supersymmetric quantum effects turn out to be also of the same sign
and could be of the same order of magnitude, the total MSSM correction to the
top quark width could potentially result in a rather large ${\cal O}(10-25)\%$
reduction of $\Gamma (t\rightarrow W^{+}\, b)$ far beyond the conventional QCD
expectations.
