Transverse sound in a magnetic field in Upt sub 3.

We have propagated transverse sound in a magnetic field in the basal plane of UPt sub 3, with the polarization vector oriented both in the basal plane and perpendicular to it. We observe a strong anisotropy in the magnetic field dependence of the attenuation for the two polarizations. Using a simple phenomenological model, we can understand the low-temperature field dependence as a natural consequence of the anisotropy with temperature in zero field reported earlier (Phys.. Rev. . Lett. 56, 1078 [1986]). However, for increasing temperatures there are significant deviations from this model. In no case do we find evidence for new superconducting phases in a magnetic field.