  We compute the one-loop corrections to the $Z \to b\bar{b}$ vertex in the
$U(1)_R$ symmetric minimal supersymmetric extension of the standard model. We
find that the predicted value of $R_b$ is consistent with experiment if the
mass of the lighter top squark is no more than 180 GeV. Furthermore, other data
combines to place a lower bound of 88 GeV on the mass of the light top squark.
A top squark in this mass range should be accessible to searches by experiments
at FNAL and LEP.
