We report observations of the radiation pressure induced expansion of a solid dielectric resonator. This effect causes a fundamental limit to the frequency stability of the resonator. The measurements were made on four high e-factor quasi-TE ''whispering gallery'' modes from 9.9 to 12.6 GHz in a monocrystalline sapphire resonator at liquid helium temperatures. The fractional frequency shift is -1.0 +/- 0.1 x 10(-12) per mu J of energy stored in the resonator. This result is consistent in sign, magnitude, and linearity with the radiation pressure induced lattice expansion term predicted by Braginsky.