  We have measured the transverse ultrasonic attenuation, $\alpha_{q\epsilon}$,
in a well-characterized single crystal of $UPt_3$ for propagation direction,
${\bf q}$, in the basal plane and polarization, $\epsilon$, in and out of the
plane. Unlike previous measurements, the sample was of sufficient purity to
exhibit three well-defined phases as a function of temperature and magnetic
field. We uncover a functional dependence for the attenuation in the (zero
field) high temperature phase (A-phase) which is distinctly different than that
in the low temperature (B) phase. Our data provide unique directional
information on the quasiparticle momentum distribution in this largely
unstudied phase of $UPt_3$ and provide the first direct evidence that the two
zero-field phases possess different symmetries.
