  We calculate the two-loop QCD correction to the scalar quark contributions to
the electroweak gauge-boson self-energies at zero momentum-transfer in the
supersymmetric extension of the Standard Model. We then derive the
$O(\alpha_s)$ correction to the contribution of the scalar top and bottom quark
loops to the rho parameter, which is the most sizable supersymmetric
contribution to the electroweak mixing angle and the W-boson mass. The two-loop
corrections modify the one-loop contribution by up to 30%; the gluino decouples
for large masses. Contrary to the SM case where the QCD corrections are
negative and screen the one-loop value, the corresponding corrections in the
supersymmetric case are in general positive, increasing the sensitivity in the
search for scalar quarks through their virtual effects in high-precision
electroweak observables.
