  The open supermembrane contribution to the non-perturbative superpotential of
bulk space five-branes in heterotic M-theory is presented. We explicitly
compute the superpotential for the modulus associated with the separation of a
bulk five-brane from an end-of-the-world three-brane. The gauge and
kappa-invariant boundary strings of such open supermembranes are given and the
role of the holomorphic vector bundle on the orbifold fixed plane boundary is
discussed in detail.
