The Hydrographic Department of Japan initiated a long-term geological and geophysical survey of the Philippine Sea in 1983. We here summarize the bathymetry and magnetic anomalies of this area, focusing on the structure and the evolutionary process of the marginal basins. The Shikoku and Parece Vela Basins developed as one backarc system in the later phase of their formation; the significant difference between the two basins, however, is clearest near the extinct spreading centers. The steep Oki-Daito Escarpment, located in the northern West Philippine Basin, was mapped for the first time during this survey. The N-S trending spreading fabric north of the escarpment is in striking contrast to the NW-SE to E-W lineament in the south. Our data reveal a more complicated history of the western Philippine Sea than has been proposed in previous studies. ¿ 1999 American Geophysical Union