  The extra contributions due to an extra generation of vector-like quarks to
the isospin symmetry breaking observable associated with the radiative B->K^*
\gamma decay is obtained. It is shown that this additional contribution is
sensitive to the nonunitarity parameter U^{sb}, which is a measure of the the
strength of the non-zero tree-level flavor changing neutral current in this
model. The significance of this result is that, once accurate experimental
resultson the isospin asymmetry becomes available, one can constrain the U^{sb}
independent of the mass of the extra quarks and so a much more stringent
acceptablemodel parameter space could be obtained.
