  The Born-Infeld theory of a toroidal D3-brane has an SL(5,Z) U-duality
symmetry. We investigate how this symmetry is reflected in the supersymmetry
algebra. We propose an action of the group on the gauge theory fields in the
BPS sector by introducing an extra field together with an additional symmetry,
and argue for the U-invariance of the degeneracies of the BPS spectrum.
