  We propose a method for constructing super-brane actions where every
background tensor potential corresponds to a world-volume field strength. The
procedure provides a natural coupling to the background and automatically
displays the SL(2;Z) symmetry of the IIB string theory. The Dirichlet 3-brane
is used as a test ground for these ideas. A polynomial action consistent with
non-linear self-duality is presented. Invariance of the action under
kappa-symmetry is demonstrated for arbitrary on-shell type IIB supergravity
backgrounds and is shown to require self-duality.
