  We report the first experimental violation of Bell's inequality in the
spatial domain using the Einstein--Podolsky--Rosen state. Two-photon states
generated via optical spontaneous parametric downconversion are shown to be
entangled in the parity of their one-dimensional transverse spatial profile.
Superpositions of Bell states are prepared by manipulation of the optical
pump's transverse spatial parity--a classical parameter. The Bell-operator
measurements are made possible by devising simple optical arrangements that
perform rotations in the one-dimensional spatial-parity space of each photon of
an entangled pair and projective measurements onto a basis of even--odd
functions. A Bell-operator value of 2.389 +- 0.016 is recorded, a violation of
the inequality by more than 24 standard deviations.
