A discontinuity exists in the moderate-depth seismicity located by the Shumagin seismic network in the eastern Aleutian subduction zone. The boundary is nearly perpendicular to the arc and has a double planed Wadati-Benioff zone to the west and a single planed zone to the east. The transition does not appear to be an artifact of mislocations, as shown by three-dimensional ray trace modeling and examination of phases of representative earthquakes. Current theory on the presence or absence of double seismic zones suggests a change in stress across the boundary. If so, the east and west regions may either have different aseismic to seismic slip ratios, or be in different stages of the earthquake cycle. ¿ American Geophysical Union 1987