  Flavor nonconserving and CP violating effects of the quark mixings are
investigated in electroweak models incorporating singlet quarks. Especially,
the D^0 - {\bar D}^0 mixing and the neutron electric dipole moment, which are
mediated by the up-type quark couplings to the neutral Higgs fields, are
examined in detail for reasonable ranges of the quark mixings and the singlet
Higgs mass scale. These neutral Higgs contributions are found to be comparable
to or smaller than the experimental bounds even for the case where the singlet
Higgs mass scale is of the order of the electroweak scale and a significant
mixing is present between the top quark and the singlet quarks.
