Anisotropy of the Magnetic-Field—Induced Phase Transition in Superconducting U<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Pt</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

Measurements of the attenuation of longitudinal ultrasound in superconducting U${\mathrm{Pt}}_{3}$ show a peak, below ${H}_{c2}$, which depends strongly on the orientation of the field relative to the $c$ axis. We propose an explanation of this peak in terms of a vortex phase transition in an unconventional superconducting state, which is consistent with recent neutron-scattering data.