Po/So phases generated by a deep-focus earthquake (focal depth: 360 km) were observed at epicentral distances between 11¿ and 14¿ in the northwestern Pacific by an ocean-bottom seismometer (OBS) array. The apparent velocity of Po was about 8.3 km/s, which is higher than is observed for shallow-focus earthquakes. The high-frequency, long-duration features of Po/So wave trains appear to be related to observed high Q's in the deep dipping seismic zone. Lateral propagation of Po/So associated with a deep-focus earthquake seems to begin when the P wave travels up the subducting slab and reaches the upper lithosphere beneath the trench axis. Also, a waveguide phenomenon occurs in the dipping subduction slab itself. Wave trains generated near the source should be taken into consideration in the theory of Po/So generation. ¿ American Geophysical Union 1990