  We compute the supersymmetric contribution to
  $R_{b}\equiv \Gamma (Z\to b{\bar b})/\Gamma (Z\to {\rm hadrons})$ in a
variety of supergravity models. We find $R^{\rm susy}_b\lsim0.0004$, which does
not shift significantly the Standard Model prediction
  ($R^{\rm SM}_b=0.2162$ for $m_t=160\GeV$). An improvement in experimental
precision by a factor of four would be required to be sensitive to such an
effect.
