We compute the one-loop corrections to the Z→bbˉ vertex in the
U(1)R symmetric minimal supersymmetric extension of the standard model. We
find that the predicted value of Rb 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