  Some aspects are considered on the flavor-nonconservation and $ CP
$-violation arising from the quark mixings with singlet quarks. In certain
models incorporating the singlet quarks, the contributions of the quark
couplings to the neutral Higgs fields may become more significant than those of
the neutral gauge interactions. Then, they would provide distinct signatures
for new physics beyond the standard model in various flavor-nonconserving and $
CP $-violating processes such as the neutron electric dipole moment, $ D^0 $-$
{\bar D}^0 $, $ K^0 $-$ {\bar K}^0 $, $ B^0 $-$ {\bar B}^0 $, $ b \to s \gamma
$, and so on.
