Onset time differences of up to 3 s are observed between transverse (SSH) and longitudinal (SSV) components of broadband S waves at distances of 85¿ to 120¿ for paths traversing the lowermost mantle (D'') beneath the Pacific. After correction for upper mantle anisotropy, SSH usually arrives earlier than SSV with the splitting increasing with distance from 100¿ to 120¿. The data yield two possible models of anisotropy: (1) anisotropy may vary laterally, with transverse isotropy existing in higher-than-average shear velocity regions beneath the northeastern Pacific, or (2) anisotropy may vary with depth, with transverse isotropy concentrated in a thin (100 km) thermal boundary layer at the base of D''. A few recordings at distances less than 105¿ show that SSV arrives earlier than SSH, indicating that general anisotropy likely exists in shallower regions of D''. ¿ 1998 American Geophysical Union