A combined location and velocity inversion technique is applied to travel time data from 151 well-recorded central Aleutian earthquakes. The data include P and S wave arrivals at stations of a local network and P, pP, and sP wave arrivals at teleseismic stations. After correction for crustal structure at the reflection points, the depth phases pP and sP provide important constraints on subduction zone structure not ordinarily resolved by other types of data. The structure is assumed to vary only across the arc and in depth; it is parameterized with cubic splines by specifying the velocity at each point of a gridded ar cross section. To avoid ray tracing, it is assumed that the velocity part of the problem is linear. Preliminary results show that deep earthquakes apparently occur within a narrow downdip zone near the upper surface of the descending slab, in contrast with previous studies which have located them within the presumably stronger and colder inner core of the slab. A slab thickness of 80--100 km and a downdip length of about 400 km, well below the deepest seismic activity, are indicated. The slab is characterized by seismic velocities as much as 11% higher than the surrounding mantle in its upper portions and 4--6% higher at depth. A sharp velocity gradient and lower velocities occur directly beneath the volcanic arc near the top of the slab. The slab anomaly appears to spread out and fall off very slowly with depth; these are probably not real effects but consequences of the omission of three-dimensional ray tracing. ¿ American Geophysical Union 1987