B̄–B mixing constrains topcolor-assisted technicolor

We argue that extended technicolor augmented with topcolor requires that all mixing between the third and the first two quark generations resides in the mixing matrix of left-handed down quarks. Then, the B̄d–Bd mixing that occurs in topcolor models constrains the coloron and Z′ boson masses to be greater than about 5 TeV. This implies fine tuning of the topcolor couplings to better than 1%.