Serpentinized peridotite is detected seismologically by mapping Poisson's ratio beneath the Kanto district, central Japan, because serpentinite has a higher Poisson's ratio than other rocks in the crust and upper mantle. We determine three-dimensional P and S wave velocity models using travel time tomography and then construct a three-dimensional map of Poisson's ratio. The models show low velocity anomalies near the upper boundary of the slab in a depth range from 20 to 70 km. In the low velocity region, there is an area with Poisson's ratio greater than 0.3 at depths of 20--45 km, which we attribute to serpentinized peridotite. The rest of the low velocity region is interpreted as descending oceanic crust with a Poisson's ratio of about 0.25. The high Poisson's ratio area is associated with low seismicity and weak interplate coupling, which is consistent with the ductility of serpentinite. ¿ 2000 American Geophysical Union