  The gravitational force on antimatter has never been directly measured. A
method is suggested for measuring the acceleration of antimatter $(\bar g)$ by
measuring the deflection of a beam of neutral antihydrogen atoms in the Earth's
gravitational field. While a simple position measurement of the beam could be
used, a more efficient measurement can be made using a transmission
interferometer. A 1% measurement of $\bar g$ should be possible from a beam of
about 100,000 atoms, with the ultimate accuracy being determined largely by the
number of antihydrogen atoms that can be produced. A method is suggested for
producing an antihydrogen beam appropriate for this experiment.
