  We use mirror symmetry to determine and sum up a class of membrane instanton
corrections to the hypermultiplet moduli space metric arising in Calabi-Yau
threefold compactifications of type IIA strings. These corrections are mirror
to the D1 and D(-1)-brane instantons on the IIB side and are given explicitly
in terms of a single function in projective superspace. The corresponding
four-dimensional effective action is completely fixed by the Euler number and
the genus zero Gopakumar-Vafa invariants of the mirror Calabi-Yau.
