  By properly accounting for the invariance of a Calabi-Yau sigma-model under
shifts of the $B$-field by integral amounts (analagous to the $\theta$-angle in
QCD), we show that the moduli spaces of such sigma-models can often be enlarged
to include ``large radius limit'' points. In the simplest cases, there are
holomorphic coordinates on the enlarged moduli space which vanish at the limit
point, and which appear as multipliers in front of instanton contributions to
Yukawa couplings. (Those instanton contributions are therefore suppressed at
the limit point.) In more complicated cases, the instanton contributions are
still suppressed but the enlarged space is singular at the limit point. This
singularity may have interesting effects on the effective four-dimensional
theory, when the Calabi-Yau is used to compactify the heterotic string.
