The longitudinal (LA) and transverse (TA) Brillouin spectra along [001] phonon direction have been measured as a function of temperature (50–475 K) in single‐crystal PbMg1/3Nb2/3O3with scattering angles &thgr;s=180° and 32.5±0.2°. The Brillouin frequency shift with decreasing temperature shows a broad softening anomaly for both LA and TA phonon modes. For &thgr;s=180°, a gradual growth in damping with maximum near 270 K is observed and is attributed to order parameter fluctuations. An additional Landau–Khalatnikov maximum is also observed atTc∼212 K. This anomaly implies a rapid growth of ferroelectric ordering nearTcand is consistent with the earlier linear birefringence results reported by Westphaletal. The elastic stiffness and compliance constants,CE11,CE44, andsE44are also determined between 200 and 370 K. ©1995 American Institute of Physics.