  Using the strong relation between coactions of a discrete group G on
C*-algebras and Fell bundles over G, we prove a new version of Mansfield's
imprimitivity theorem for coactions of discrete groups. Our imprimitivity
theorem works for the universally defined full crossed products and arbitrary
subgroups of G, as opposed to the usual theory which uses the spatially defined
reduced crossed products and normal subgroups of G. Moreover, our theorem
factors through the usual one by passing to appropriate quotients. As
applications we show that a Fell bundle over a discrete group is amenable in
the sense of Exel if and only if the double dual action is amenable in the
sense that the maximal and reduced crossed products coincide. We also give a
new characterization of induced coactions in terms of their dual actions.
