  The production of $D_s^-$ relative to $D_s^+$ as a function of $x_F $ with
600 GeV/c $\Sigma^-$ beam is measured in the interval $0.15 < x_F < 0.7$ by the
SELEX (E781) experiment at Fermilab. The integrated charge asymmetries with 600
GeV/c $\Sigma^-$ beam ($0.53\pm0.06$) and $\pi^-$ beam ($0.06\pm0.11$) are also
compared. The results show the $\Sigma^-$ beam fragments play a role in the
production of $D_s^-$, as suggested by the leading quark model.
