  We evaluate non factorizable $O(\alpha\alpha_s)$ corrections to the process
$Z\to b\bar{b}$ due to the virtual t-quark. All two-loop vertex diagrams with
$W$'s and charged ghosts $\Phi$'s are included. They are evaluated in the large
top-mass expansion up to the $10^{th}$ order. Gluon Bremsstrahlung is taken
into account by integrating over the whole phase space. All calculations,
including Bremsstrahlung, are done in dimensional regularization. The expansion
coefficients of the large mass expansion are given in closed form. Their
expansion in $y=m_Z^2/4m_W^2$ is in agreement with the coefficients up to
$O(m_W^6/m_t^6)$ as given by Harlander et al.
