Sound velocities of olivine (&agr;) and beta (&bgr;) polymorphs of Mg2SiO4 are measured to P>12 GPa at room temperature on polycrystalline samples in a multi-anvil apparatus using ultrasonic interferometry. The new velocity data for olivine are lower by 1% for P waves and 2% for S wave at transition zone pressures than extrapolations of low pressure (<1 GPa) data. However, the elastic bulk (KS) and shear (G) moduli for the polycrystalline olivine exhibit good agreement to 10 GPa with recent Brillouin scattering and ultrasonic data for single crystals. The velocity data for the polycrystalline beta phase at 12 GPa agree within 1% with Eulerian finite strain extrapolations of previous data by Gwanmesia et al. (1990b) for similar specimens. The pressure derivatives of the elastic moduli calculated by linear fittings to KS and G vs. pressure yield K0'=4.4 and G0'=1.3 for the olivine and K0'=4.2 and G0'=1.5 for the beta phases. The isothermal velocity contrast at room temperature between these two phases, (V&bgr;-V&agr;)/V&agr;, decreases systematically with increasing pressure, reaching 8.0% for P wave and 11.3% for S wave at pressures equivalent to 410 km depth. ¿ American Geophysical Union 1996