  The SL(2,Z) duality symmetry of IIB superstring is naturally realized on the
D=11 supermembrane restricted to have central charges arising from a nontrivial
wrapping. This supermembrane is minimally immersed on the target space (MIM2).
The hamiltonian of the MIM2 has a discrete quantum spectrum. It is manifestly
invariant under the SL(2,Z) symmetry associated to the conformal symmetry on
the base manifold and under a SL(2,Z) symmetry on the moduli of the target
space. The mass contribution of the string states on the MIM2 is obtained by
freezing the remaining degrees of freedom. It exactly agrees with the
perturbative spectrum of the (p,q) IIB and IIA superstring compactified on a
circle. We also construct a MIM2 in terms of a dual target space, then a (p,q)
set of non-perturbative states associated to the IIA superstring is obtained.
