  CP asymmetry in $B \to \phi K_S$ decay is studied in a special context of
supersymmetry theories, in which the charginos play an important role. We find
that in addition to the gluino, chargino can also make large contributions to
CP asymmetry in $B \to \phi K_S$ decay. After considering the constraints from
$B \to J/psi K_S$ decay, we study three special scenarios: (a). Large mixing on
left-handed charm and top squarks (LL mixing); (b). Large mixing on
right-handed charm and top squarks (RR mixing); (c). Large mixing on
left-handed charm and top squarks plus right-handed charm and top squarks (LL +
RR mixing). We show quantitatively that because of large squark mixing within
second and third generations, an $\mathcal O$(1) effect on CP violation in $B
\to \phi K_S$ is possible.
