Measurements with the Energetic Particles Detector (EPD) on Galileo orbit C9 in the Jovian magnetotail revealed the existence of distinct quasi-periodic variations of energetic ion intensities which are superimposed on the well-known 10-hour modulations due to the planetary rotation. The intensity variations are associated with changes of the particle energy spectra and the plasma flow pattern. They are clearly of temporal nature and not the consequence of the spacecraft passing through periodically separated spatial structures. The modulation period is about 3 days. The oscillations are most pronounced throughout the middle magnetotail regime (20 to 80 RJ), however, seem to persist even in the deep tail region. The amplitude of the modulation is dependent on the particle energy. The highest energies measured (about 1 MeV) show the strongest variations. Energetic particle features with similar periodicity are observed on other Galileo orbits as well. The cause of these modulations is unclear; however, it may be speculated that they correspond to a quasi-periodic transition between two basic states of the Jovian magnetotail which occur with a time constant inherent to the Jovian magnetosphere. ¿ 1998 American Geophysical Union