The juxtaposition of a liquid iron-dominant alloy against a mixture of silicate and oxide minerals at Earth's core-mantle boundary is associated with a wide range of complex seismological features. One category of observed structures is ultralow-velocity zones, which are thought to correspond to either aggregates of partially molten material or solid, iron-enriched assemblages. We measured the phonon dispersion relations of (Mg,Fe) O magnesiowüstite containing 76 mol % FeO, a candidate ultralow-velocity zone phase, at high pressures using high-energy resolution inelastic X-ray scattering. From these measurements, we find that magnesiowüstite becomes strongly elastically anisotropic with increasing pressure, potentially contributing to a significant proportion of seismic anisotropy detected near the base of the mantle. ©2018. American Geophysical Union. All Rights Reserved.