A MASSIVE star that explodes as a supernova produces an expanding remnant-a shell of gas-and leaves behind a neutron star. It has been suggested that instabilities during these explosions may create high-density clumps of ejecta(1,2), but such fragments have never been found in a remnant containing a pulsar. Here we present X-ray and radio observations of a feature outside the shack-wave boundary of the Vela supernova remnant (associated with the pulsar PSR0833 - 45), which appears to be an ejected fragment, with a wake(3). The feature, which has travelled nearly 50 pc from the site of PSR0833 - 45 but is only one parsec across, is a clearcut case of a ’bullet’ of ejecta moving supersonically through the surrounding medium. The radio observations show non-thermal emission along the leading edge of the X-ray feature, indicating particle acceleration at the fragment’s shock front.