  We show that tree level ``resonant'' $N$ tachyon scattering amplitudes, which
define a sensible ``bulk'' S -- matrix in critical (super) string theory in any
dimension, have a simple structure in two dimensional space time, due to
partial decoupling of a certain infinite set of discrete states. We also argue
that the general (non resonant) amplitudes are determined by the resonant ones,
and calculate them explicitly, finding an interesting analytic structure.
Finally, we discuss the space time interpretation of our results.
