  We analyze the vector multiplet prepotential of d=4, N=2 type IIA
compactifications. We find that the worldsheet instanton corrections have a
natural interpretation as one-loop corrections in five dimensions, with the
extra dimension being compactified on a circle of radius $g_{s}\ell_{s}$. We
argue that the relation between spacetime and worldsheet instantons is natural
from this point of view. We also discuss the map between the type IIA
worldsheet instantons and the spacetime instantons in the heterotic dual.
