  A supersymmetric $Sp_{L}(6) \times U_{Y}(1)$ model (SUSY $Sp(6)$) is proposed
as an extension of the standard electroweak model. The model is applied in a
phenomenological study of $B^{0}_{d} \bar{B}^{0}_{d}$ mixing. It is found that
the supersymmetric (SUSY) partner $\tilde{z}^{\prime}$ of the extra
$Z^{\prime}$ can significantly cancel the other contributions to bring the
mixing parameter $x_{d}$ within the experimentally allowed range $0.57
\stackrel{<}{\sim} x_{d} \stackrel{<}{\sim} 0.77$ for a top mass of $158
\stackrel{<}{\sim} m_{t} \stackrel{<}{\sim} 194$ Gev. Other interesting and
possibly novel features of flavor changing neutral currents (FCNC) in SUSY
theories with horizontal gauge symmetries are pointed out.
