The stability and P-V equation of state of CaSiO3 have been investigated using in situ diamond-anvil X ray diffraction techniques to 134 GPa, a pressure equivalent to that at the core-mantle boundary. Samples were heated by YAG laser at each pressure increment at high pressures to accelerate phase transitions. X ray diffraction measuremnts were carried out at 300 K using both energy-dispersive synchrotron and sealed-tube film techniques. Quenched CaSiO3-perovskite was observed to remain metastable close to 0.1 MPa, and to convert rapidly to an amorphous phase on pressure release. The simple cubic perovskite phase of CaSiO3 was found to be the stable phase for all lower mantle pressure conditions. All 47 P-V data points were used to obtain a third-order Birch-Murnaghan equation of state with zero-pressure parameters: unit cell volume V0=45.37¿(0.08) A⊙3, density &rgr;0=4.252(¿0.008) Mg/m3, and bulk modulus K'0=281(¿4) GPa, with an assumed bulk modulus pressure K0=4. These parameters are close to those of (Mg0.88Fe0.12)SiO3-perovskite and to those inferred by the Preliminary Reference Earth Model for the lower mantle. Hence, CaSiO3 must be considered an invisible component, in terms of density and bulk modulus constraints, in the lower mantle. Mantle composition models with both high and low calcium content can satisfy existing seismological constraints for the lower mantle. ¿ American Geophysical Union 1989