  We study the effect of new physics on the Z-decay into b anti-b pairs in the
framework of 331 models. The decay Z -> b anti-b is computed at one loop level
and, using previous results, we evaluate this branching fraction in the
framework of 331 models. A wide range of the space parameter of the model is
considered and possible deviations from the standard model predictions are
explored. From precision measurements at the Z-pole we find the allowed region
for MJ3, MX at 95 % CL.
