To determine the location, strength, and orientation of seismic anisotropy in the Mariana subduction zone beneath Guam, we evaluated shear wave splitting in local S, regional S and ScS, and teleseismic core phases such as SKS recorded at station GUMO. Fast directions from the local S phases have an average azimuth of -45¿, and splitting times range from 0.1 s to 0.4 s. For local S phases from events within the southeastern half of the subducting slab, splitting parameters manifest minimal frequency dependence in both fast direction and splitting time. However, for the remaining local S phases in the data set, fast directions vary with frequency content. No well-constrained splitting parameters were obtained for the regional and teleseismic phases, but the particle motions of these unsplit phases are consistent with an average anisotropic fast direction of ~-45¿. Anisotropy due solely to olivine oriented by slab-entrained flow in the mantle wedge would produce local S fast directions at ~-66¿, and anisotropy due solely to fossil seafloor spreading in the subducting slab would yield fast directions at -20¿ to -30¿. Neither of these predictions is consistent with the observed fast directions. However, the observed splitting, including the frequency-dependent fast directions, can be explained by models containing anisotropy in both the slab and wedge, and possibly (although not necessarily) anisotropy due to recent extension in the overriding Philippine sea plate. ¿ 1998 American Geophysical Union