We systematically study the energetic particle (~25 to 850 keV) dawn-dusk plasma sheet asymmetry using both ion and electron spectra composed of 56 energy channels. These spectra, corresponding to an average duration of ~8 hours, have been provided by the Interball tail probe and cover the extent between 15 and 28 RE away from the Earth. The events are classified in four Y coordinate categories: the dawnside with YGSMY>-13 RE, the center dusk with 013 RE. The asymmetry is profound between the two extreme dawn-dusk flanks and weaker between the two central plasma sheet regions. In all the energy channels the dusk flank ion fluxes exceed those of electrons. At the dawnside the energetic electron fluxes frequently outnumber those of ions above a bottom energy threshold. During quite times (Kp<2) the ion (electron) fluxes in the duskside (dawnside) plasma sheet exceed by far those in the dawn (dusk) flank. In general, the ion and electron fluxes significantly increase as the Kp index steps upward. However, owing to the dawn-dusk asymmetry, one can observe higher electron fluxes at dawn with low Kp than at dusk with high Kp. On the basis of two case studies, it is shown that ongoing substorm processes and plasma temperature transitions temporarily affect the spatial dawn-dusk asymmetry and the associated spectra. In one case study, via successive spectra it was concluded that the betatron acceleration mechanism is probably the dominant energization process, although the observed spectra did not preserve their spectral shape. This is interpreted as due to the action of energy-dependent large-scale drifts across the tail that strongly modify the initial source spectra. The dawn-dusk asymmetry and all the spectral characteristics from inside the plasma sheet are transmitted out into the magnetosheath, and this may help us (in the future) to discriminate between merging and leakage processes taking place over the magnetopause. ¿ 2001 American Geophysical Union