Using untagged B⁰ -> D K_S to determine gamma

It is shown that the weak phase gamma=arg(-V_{ud}V^*_{ub}V_{cb}V_{cd}^*) can be determined using only untagged decays B/Bbar--> D K_S. In order to reduce the uncertainty in gamma, we suggest combining information from B^{+-}--> DK^{+-} and from untagged B^0 decays, where the D meson is observed in common decay modes. Theoretical assumptions, which may further reduce the statistical error, are also discussed.