Interpretation of a reversed seismic-refraction profile recorded between American Falls reservoir and Flaming Gorge reservoir in May 1963 indicates that the depth to the Mohorovicic discontinuity is about 31 km at American Falls and 37 km at Flaming Gorge. The existence of an intermediate crustal layer at a depth of about 19 to 21 km beneath the profile is well supported by refractions and reflections. The velocity of compressional waves in the mantle just beneath the Mohorovicic discontinuity is about 7.8 km/sec, their velocity in the intermediate layer is about 6.9 km/sec, and their velocity in the upper crust (beneath the near-surface low-velocity material) is about 5.9 km/sec. A prominent phase with an apparent velocity of 8.4 km/sec was recorded at distances of 210 km to 325 km from shots at American Falls. This phase is believed to be a reflection from a boundary within the mantle.