A detailed description of a plasmoid is presented on plasma electron data, energetic particle data, and magnetic field data from ISEE 3. The plasmoid, 75-150 RE long, traveling tailward at 500-1000 km/sec, appeared at ISEE 3, ~220 RE down-tail, 27 minutes after a very strong intensification of the auroral electrojet was recorded at several ground stations. The plasmoid was overlaid by a ''separatrix layer'' within which energetic particles, probably originating at a near-earth reconnection region, streamed freely tailward along the field lines, the slower particles being encountered deeper within the layer. The plasmoid contained plasma of uniformly high temperature and its boundary was ≲1 RE thick.