Non-singlet corrections of O(alpha(2)(s)G(F)M(t)(2)) to Gamma(Z -> b(b)over-bar)

The partial decay rate of the Z boson into bottom quarks constitutes an imp ortant decay channel. This is mainly due to the virtual presence of the top quark in the loop diagrams giving rise to correction factors which are qua dratic in the top quark mass. At one- and two-loop order it turned out that the leading term in the heavy-top expansion leads to very good approximati ons to the exact result. In this;work the non-singlet diagrams at O(a(s)(2) G(F)M(t)(2)) alt considered.