  New physics associated with the heavy top quark can affect top quark
production and the partial decay width of $Z \rightarrow b \bar b$. In this
paper, we examine the correlated effects of possible new physics on ${R_b}$
measured at LEP I and the single top quark production rate at Tevatron by using
an effective lagrangian technique. We point out that certain operators in the
effective lagrangian, constrained by the measured value of $R_b$, can lead to
significant and potentially observable effects in single top production.
