Seismic structure of the upper mantle transition zone beneath the Philippine Sea region is studied using waveform modeling. Models AZ41 and AZ40 were derived to represent the structure beneath the north central part and northwestern regions, respectively. AZ41 has high-velocity anomalies from 510 km to 660 km 3% faster than a global model ak135, a depression of the '660' discontinuity to 690 km, and a small (+2%) discontinuity at 510 km. Model AZ40 is the same as AZ41 except there is no depression of the '660'. On the other hand, the waveforms which sampled the southern region can be explained with the global model ak135. These results support the interpretation that subducted Pacific lithosphere is stagnant above the '660' in the northern part, whereas in the southern part it penetrates into the lower mantle. In accord with earlier suggestions, the different configuration of the subducted slab suggested above could be due to the migration of the trench. ¿ 2001 American Geophysical Union