All elastic constants, the average bulk and shear moduli, and the lattice parameters of San Carlos olivine (Fo90) (initial density 3.355 gm/cm3) have been determined to a pressure of 12 GPa at room temperature. Measurements of c11, c33, c13, and c55 have been extended to 17 GPa. The pressure dependence of the adiabatic, isotropic (Hashin-Shtrikman bounds) bulk modulus, and shear modulus may be expressed as KHS=129.4+4.29P and by GHS=78+1.71P-0.027P2, where both the pressure and the moduli are in gigapascals. The isothermal compression of olivine is described by a bulk modulus given as KT=126.3+4.28P. Elastic constants other than c55 can be adequately represented by a linear relationship in pressure. In the order (c11,c12,c13,c22,c23,c33,c44,c55,c66) the 1 bar intercepts (gigapascal units) are (320.5, 68.1, 71.6, 196.5, 76.8, 233.5, 64.0, 77.0, 78.7). The first derivatives are (6.54, 3.86, 3.57, 5.38, 3.37, 5.51, 1.67, 1.81, 1.93). The second derivative for c55 is -0.070 GPa-1. Incompressibilities for the three axes may also be expressed as linear relationships with pressure. In the order of a, b, and c axes the intercepts in gigapascals are (547.8, 285.8, 381.8) and the first derivatives are (20.1, 12.3, 14.0).¿ 1997 American Geophysical Union