Seismic waves arriving between the P and S phases have been observed at South American seismograph stations from large local deep-focus earthquakes. The travel times, frequency content, and amplitudes for these waves are consistent with their being sp phases: S waves which convert to P waves upon refraction across an approximately horizontal interface at a depth of 400 km. The velocity change across the interface is at least 5% over a transition zone of at most a few kilometers. The polarity of the sp indicates that there is a velocity reversal below this interface. We therefore suggest that the discontinuity may be the lithosphere-asthenosphere boundary.