Broadband seismograms recorded by the seismic stations deployed on oceanic islands in the South Pacific for two deep earthquakes in 1998 are used to investigate the mantle transition zone structure beneath the South Pacific, where a large-scale hot plume might ascend from the core-mantle boundary (CMB). By stacking S waves reflected at the mantle transition zone discontinuities, we find clear signals associated with the 410-km and 660-km discontinuities. The transition zone thickness, which is constrained from the travel time difference between the reflected waves from the 410-km and 660-km discontinuities, is observed to be approximately 15 km thinner beneath the South Pacific Superswell than that beneath other regions. The result suggests that the transition zone temperature beneath French Polynesia is approximately 100~200 K higher than the surrounding mantle. ¿ 2000 American Geophysical Union