  In this work, we update the constraints on tree level FCNC couplings in the
framework of a theory with n isosinglet vector-like down quarks. In this
context, we emphasize the sensitivity of the B -> J/psi K_S CP asymmetry to the
presence of new vector-like down quarks. This CP asymmetry, together with the
rare decays B -> X_{s,d} l bar{l} and K -> pi nu bar{nu} are the best options
to further constrain the FCNC tree level couplings or even to point out, in the
near future, the possible presence of vector-like quarks in the low energy
spectrum, as suggested by GUT theories or models of large extra dimensions at
the TeV scale.
