Observations of shear‐wave splitting were obtained from temporary and permanent stations in western Nevada and southern California. Measurements of fast polarization azimuth ϕ and delay time δtwere made on both SKS and direct S phases. For stations in the northern Basin and Range the results were consistent, yielding an average of (ϕ, δt) = (+75 ± 8°, 0.9 ± 0.3 s). One station in the southern Basin and Range gave (+40 ± 11°, 0.4 ± 0.1 s) and one in the Mojave Desert gave (−54 ± 3°, 1.2 ± 0.1 s). δt= 0.9sis consistent with a 100km‐thick mantle layer characterized by 4% anisotropy. For the northern stations, the anisotropy appears unrelated to the present‐day extension or absolute plate motion. Rather, we suggest that it is ‘fossil’ anisotropy associated with pre‐Miocene extension, whose direction is about +68°. For the California station ϕ is nearly parallel to the strike of the San Andreas Fault system, and is attributed to shear strain associat