  We rewrite the SL(2,Z) covariant worldsheet action for the IIB string
proposed by Townsend in a Polyakov form. In a flat background the formalism
yields separate (p,q) sectors. In each one the action is that of the IIB string
action with the string slope parameter \alp\pr replaced by its SL(2,Z) analogue
\alp_{pq}\pr. SL(2,Z) invariant graviton scattering amplitudes are obtained
from those of the fundamental (1,0) string by summing over the different
sectors. The tree-level four-graviton amplitude in this formalism differs from
a previously conjectured non-perturbative form; both yield the same expansion
in order \alpha\pr^3.
