Room temperature neutron inelastic scattering measurements of the polar transverse optic (TO) phonon mode in the cubic relaxorPb(Mg1/3Nb2/3)O3(PMN) reveal anomalous behavior, similar to that recently observed inPb(Zn1/3Nb2/3)0.92Ti0.08O3,in which the optic branchappearsto drop precipitously into the acoustic branch at a finite value of the momentum transferq=0.23&hthinsp;Å−1,measured from the zone center. By contrast, a recent neutron study indicates that PMN exhibits a normal TO phonon branch at much higher temperature (800 K). We thus speculate that this unusual feature is common to all relaxor materials at low temperatures, and is the result of the presence of nanometer-scale polarized domains in the crystal that form below a temperatureTd,which effectively prevent the propagation of long wavelength(q=0)phonons. ©2000 American Institute of Physics.