Anomalously high rates of crustal deformation have been measured at the Basin-Range transition to the Rocky Mountains along the Wasatch fault zone, Utah, by repeated Global Positioning System (GPS) measurements. Four GPS field campaigns (1992--1995) and comparisons with older (1962--1991) geodetic data have revealed east-west extensional strain at a rate of 0.05¿0.02 &mgr;strain/yr, corresponding to a 2.7¿1.3 mm/yr rate of horizontal displacement across a 55-km wide area. This rate is more than 20% of the total ~12 mm/yr extension rate across of the ~800-km wide Basin and Range province. It is also two to three times larger than the average Late Quaternary fault slip rate on the Wasatch fault and tens of times larger than the displacement rates inferred from the cumulative seismic moments of historic earthquakes. While we do not yet know the source of this unexpected contemporary deformation, possible mechanisms include homogeneous crustal extension, loading of the Wasatch and adjacent faults, and pressure solution creep. If the Wasatch fault is being loaded by this high strain rate, it increases the expected peak ground acceleration significantly from standard values. These new findings demonstrate the importance of GPS in earthquake hazard assessment. ¿ 1998 American Geophysical Union