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Ultrasonic measurements of compressional and shear wave velocities are reported for peridotite, dunite, partially serpentinized dunite and peridotite, and serpentinite at hydrostatic pressures to 10 kb. Compressional wave velocities for monomineralic aggregates of olivine, pyroxene, and serpentine approximate 8.54, 7.93, and 5.10 km/sec, respectively. Shear wave velocities for similar aggregates are 4.78, 4.65, and 2.35 km/sec. The relationships between velocity and mineralogy are reported for rocks containing olivine, pyroxene, and serpentine. Young's modulus, Lamé's constant, the bulk modulus, and the shear modulus of peridotites and dunites decrease rapidly with serpentinization. Poisson's ratio and compressibility increase with serpentinization. The effect of olivine orientation on velocity is discussed and found to account for compressional wave anisotropy. High velocities correspond to propagation parallel to maximum concentrations of olivine a axes, and low velocities are characteristic of propagation parallel to b axes.