Ultrasonic interferometry at near-gigahertz frequencies is used to investigate longitudinal sound wave velocities parallel to [100> and [111> and the elastic constant c11 for high quality single crystals of magnesiow¿stite (Mg, Fe)O. The measurements were made at room pressure and temperature on samples spanning the full compositional range from periclase (MgO) to w¿stite (Fe1-xO). A strong P-wave anisotropy of ~10% was measured for periclase, increasing about 1% for Mg0.94Fe0.06O then decreasing linearly to ~0% for w¿stite. The elastic modulus c11 of magnesiow¿stite decreases linearly with iron content by -0.72(3) GPa/mol% FeO with the exception of a sharp increase for pure periclase. Thus, the addition of a small amount of iron to periclase has a major effect on its elastic behavior. This observation indicates that the use of a simple interpolation of elastic constant data between periclase and w¿stite [Jackson, 1998> may yield erroneous predictions for lower mantle compositions based on seismic data. ¿ 2000 American Geophysical Union