COCORP seismic reflection data show that Cenozoic extension beneath the Sevier Desert has had movement along a major, west-dipping surface. Reflections were obtained from several stratigraphic zones of known age within the Sevier Desert basin. The geometric relationships of these zones indicate that extension has occurred in at least two stages. A large first stage of movement (20-30 km or 12-18 mi) began after the middle Oligocene (26-28 Ma) and continued into the Miocene. This activity was followed by a probably short period of quiescence and erosion. A later episode of movement of smaller proportions (< 8 km or < 5 mi) began during or after the eruption of a middle Pliocene (4.2 Ma) basalt. This later stage is still active, as indicated by fault scarps which offs t recent sediments in the basin. The record of extension within the Sevier Desert reflects the history of movement along the underlying Sevier Desert detachment, a major, low-angle normal fault that has been traced over 70 km (43 mi) perpendicular to strike and to depths of 12-15 km (7.5-9.3 mi). The record of displacement along the Sevier Desert detachment may have implications for the general history of extension in the Basin and Range Province.