We perform travel time analyses for the Kuril and the Mariana subduction zones to investigate whether a lower mantle high-velocity zone exists immediately beneath the deepest earthquakes in these regions. We use P and PKIKP wave arrival time data read directly and accurately from WWSSN short-period vertical component records and corrected for the effect of lateral heterogeneity far from the source regions using a differential residual sphere technique. The corrected residuals for the deep Kuril events show that a high velocity zone extends in the down-dip direction of the Kuril slab in the lower mantle. On the other hand, the residuals for the deep Mariana events show no evidence for a high-velocity zone in the down-dip direction beneath the deepest earthquakes. A reasonable interpretation of these results is that the Kuril slab penetrates into the lower mantle (h≥670 km) but that the Mariana slab does not extend vertically into the lower mantle. ¿ American Geophysical Union 1992