Occurrence of a prominent peak in the ion energy spectrum at energies of 0.1 to 0.7 MeV is observed by the IMP-8 spacecraft during an energetic particle burst in a plasma ''dropout'' interval at about 37 Re in the earth's magnetotail. This unusual characteristic in the spectrum is detected for about 2 minutes. By fitting the observed ion spectra to a drifting Kappa distribution, it is found that the ion population can be described as a hot beam with number densities 4¿10-7 to 5¿10-4 cm-3 and temperatures 15 to 45 keV, jetting tailward at speeds of 3500 to 7000 km/s. The energy flux density associated with the beam can be as high as 8¿10-2 ergs cm-2 sec-1.