Phase travel-times for Love and Rayleigh waves across the Pacific Basin were inverted to obtain models of the upper mantle of the Pacific Basin. Inversion of the Rayleigh wave data yielded a model with S-velocities in the lid and channel of 4.597¿.018 and 4.103¿.016 km/sec, respectively, while inversion of the Love wave data resulted in lid and channel S-velocities of 4.634¿.050 and 4.244¿.031 km/sec respectively (95% confidence level). To within the indicated uncertainties, these results suggest a model with a lid which is isotropic and a channel which is anisotropic with respect to S-wave velocity. No significant evidence of azimuthal anisotropy was found. Anisotropy in the LVC and its absence in the lid is presumed to be associated with preferred orientation of partially molten material in the LVC. One model of partial melting adequate to explain the results is a configuration of small penny-shaped cracks of zero volume and radius a. The number density of spheres of radius a, N4&pgr;a3/3V, each containing one randomly oriented crack is 71%; the number density of such spheres containing one horizontal crack is 12%. The preferred orientation of cracks may be related to a shear flow gradient in the channel.