Plasmoids are closed magnetic-loop structures with entrained hot plasma which are inferred to occur on large spatial scales in space plasma systems. In this paper we present a small-scale plasmoid interpretation of one of the magnetotail barium releases which occurred during the Active Magnetospheric Particle Tracer Explorers (AMPTE) program. Using information for the May 13, 1985, event, it is shown that the AMPTE barium release at 0517 UT occurred on closed magnetic field lines of the thinned plasma sheet. A magnetospheric substorm sequence was in progress at this time. The barium rapidly ionized and presumably heated over the course of the next half hour, loading the plasma sheet flux tubes locally. Direct ground-based imaging, Doppler, and Fabry-Perot triangulation data show that ~33 min after the release, the Ba+ cloud brightened and moved off down the tail (antisunward) at high speed (≥200 km/s). We offer an explanation in terms of a two-step process in which the Ba+ ions expanding along the plasma sheet field lines cause a stretching of the field due to a mass loading effect. Then, we suggest the development of an enhanced ion tearing mode due to the local presence of the heavy (Ba+) ions. The result of these processes would be the formation of a small plasmoid structure in the thin, residual plasma sheet late in the substorm sequence. As is the case for larger plasmoids, the small-scale plasmoid would move down the tail at high speed impelled by tensional forces in the open field lines enveloping it. These results thus suggest that a plasmoid was directly observed in the AMPTE experiment by means of the optical emissions of entrained tracer ions contained in the plasma bubble. ¿ American Geophysical Union 1989