  We present a geometric formulation of $(p,q)$-strings in which the
$Sl(2;Z)$-doublet of the two-form gauge potentials is constructed as second
order in the supersymmetric currents. The currents are constructed using a
supergroup manifold corresponding to the $(p,q)$-string superalgebra, which
contains fermionic generators in addition to the supercharges and transforms
under the $Sl(2;Z)$. The properties of the superalgebra and the generalizations
to higher $p$-branes are discussed.
