  We show that in the supersymmetric standard model (SSM) box diagrams mediated
by charginos and up-type squarks may give large contributions to $\BB$ and
$\KK$ mixings. This is due to the fact that a heavy top quark mass allows large
mass differences among the up-type squarks, which leads to a less efficient
cancellation among the different squark diagrams. If the ratio of the vacuum
expectation values of the Higgs bosons is of order of unity, box diagrams
mediated by charged Higgs bosons also contribute appreciably. For sizable
regions of the SSM parameter space, the SSM contributions could be sufficiently
large to affect significantly the evaluation of the CKM matrix from the
experimental values of the mixing parameter $x_d$ and the $CP$ violation
parameter $\epsilon$.
