A distinguishing characteristic of deep earthquakes has been the absence of observable aftershock sequences(1,2). Here we report the first extensive deep-earthquake aftershock sequence to be observed; it was recorded by an array of eight broadband seismographs following the 9 March 1994 deep Tonga earthquake. The aftershocks show a power-law decay with time following the main shock, as is typical of shallow events. Most of the well located aftershocks are concentrated along a steeply dipping plane consistent with one of the nodal planes of the main-shock mechanism and the mechanisms of three large aftershocks. Assuming these aftershocks denote the main-shock rupture area, they define a 50 x 65 km fault plane extending across the entire width of the active seismic zone and into the surrounding aseismic region. The width of the aftershock zone is wider than the expected width of the metastable olivine sedge, demonstrating that either the width of the metastable olivine material exceeds previous estimates, or the aftershocks are not confined in such a wedge.