The elastic constants, average bulk and shear moduli, and lattice parameters of Kilbourne Hole orthopyroxene <(Mg1.63Fe0.17Ca0.04Mn0.01)(Al0.12Cr0.01)(Si1.89Al0.11)O6> have been determined to a pressure of 12.5 GPa. The molar ratio Mg/(Mg+Fe) in the sample is 0.91 and 5 wt % of Al2O3 and 1 wt % of CaO are present. The bulk modulus, in gigpascals, expressed as a function of the pressure (with an rms misfit of 0.5%) is K=115.4+7.82P-0.18P2. The unusually large pressure derivative of bulk modulus at 1 bar decreases to a more typical value (~4) at high pressure. The isotropic shear modulus is linear within experimental uncertainty (the rms misfit is 0.5%) with G=77.9+1.44P. At 1 bar the isotropic longitudinal and shear velocities 8.15 and 4.86 km/s, respectively are approximately 3% faster than those found in nonaluminous orthopyroxenes. Above 4 GPa the average compressional wave velocities in the orthopyroxene exceed those in olivine. These results call into question the assumption, common in models created to interpret mantle seismic data, that high velocities in garnet are balanced by low velocities in orthopyroxene.¿ 1997 American Geophysical Union