  The vertex corrections to the leptonic partial widths of the $Z$ induced by
leptoquarks that couple leptons to the top quark are considered. We obtain
stringent bounds on the parameter space of the masses and Yukawa couplings of
these leptoquarks, using the latest information on the $Z \rightarrow l^+l^-$
decay widths measured at LEP. Leptoquarks coupling with electroweak strength to
top quarks are constrained to be heavier than several hundred GeV, at 95$\%$
C.L. As a consequence, such leptoquarks cannot make a significant contribution
to lepton asymmetries, $\tau$ polarisation asymmetries or $A_{LR}$.
