We analyze the Zff vertex in the framework of models that add a new bottom quark in a non-sequential way and we evaluate the tree level contribution to the LEP/SLC observables ΓZ, Rb and Rl. We obtain bounds for the mixing angles from the experimentally allowed contour regions of the parameters ΛL,R introduced here. In order to get a more restrictive region, we consider the experimental results for B-->ννX as well. .