Improved instrumentation and revised travel-time tables for the phase P′ have made feasible a more detailed study of the phase P′P′. In many instances energy is observed preceding P′P′, sometimes by nearly 70 sec, that can be interpreted only as reflections from discontinuities or inhomogeneities in the upper mantle. For each of the regions studied, the early reflections show a degree of consistency for a number of earthquakes, but there is less consistency from region to region. In most areas there is evidence for reflections from a depth of 65 to 70 km, that could correspond to the top of the low-velocity channel. There is also fair evidence for reflecting surfaces at depths of between 110 and 140 km under Siberia, and between 150 and 190 km under western Europe. Of the regions studied, Antarctica alone showed no reflections from depths greater than the Mohorovičić discontinuity. This may indicate a more uniform mantle structure under Antarctica than under other regions. The relative amplitudes of the early reflections and the main phase are consistent with what would be expected from inhomogeneities postulated in recent mantle models.