  In this paper we prove uniqueness for an inverse boundary value problem for
the magnetic Schr\"odinger equation in a half space, with partial data. We
prove that the curl of the magnetic potential $A$, when $A\in
W_{comp}^{1,\infty}(\ov{\R^3_{-}},\R^3)$, and the electric pontetial $q \in
L_{comp}^{\infty}(\ov{\R^3_{-}},\C)$ are uniquely determined by the knowledge
of the Dirichlet-to-Neumann map on parts of the boundary of the half space.
