  We consider scattering processes involving N gluonic massless states of open
superstrings with certain Regge slope alpha'. At the semi-classical level, the
string world-sheet sweeps a disk and N gluons are created or annihilated at the
boundary. We present exact expressions for the corresponding amplitudes, valid
to all orders in alpha', for the so-called maximally helicity violating
configurations, with N=4, 5 and N=6. We also obtain the leading O(alpha'^2)
string corrections to the zero-slope N-gluon Yang-Mills amplitudes.
