A recent and rapid increase since the mid-1970's in commercial and residential development in the Powder River Basin, Wyoming and Montana, is caused by exploitation of vast coal and other resources in the basin. One geologic hazard to such development is landsliding. A landslide sufficiently representative of others in the area was chosen for detailed seismic studies. Studies of this landslide show that a low-velocity layer overlies a high-velocity layer both on the slide and away from it and that the contact between the volocity layers is nearly parallel with the preslide topographic surface. Computed shear and other elastic moduli of the low-velocity layer are about one-tenth those of the high-velocity layer. When failure occurs within the slope materials, it will very likely be confined to the low-velocity layer. The number and position of main shear planes in the landslide are unknown, but the main slippage surface is probably near the contact between the low- and high-velocity layers. The main cause of landslide failure in the study area is apparently the addition of moisture to the low-velocity layer.