  In this note, we study D-brane instantons which intersect N=1 supersymmetric
configurations of space-filling D-branes in general Calabi-Yau
compactifications of type IIB string theory. Our focus is on rigid ``stringy
instantons'' -- those which cannot be interpreted as Yang-Mills instantons in a
non-Abelian gauge group on any stack of space-filling D-branes. We show that
their contributions to the space-time superpotential can be determined by a
topological B-model computation of the tree-level superpotential for a related
auxiliary brane system. This computation is very tractable, as it is governed
by an A-infinity structure. We summarize the stringy instanton contribution to
the space-time superpotential in a simple formula.
