From August 10 to October 22, 1996, the Galileo spacecraft made observations in the midnight-to-dawn sector of the magnetosphere of Jupiter out to a distance of 124RJ, encompasing the region previously explored by the outbound passes of the Pioneer 10, Voyager 1, and Voyager 2 spacecraft. We use the count rates of electrons with energy between 1 and 4 keV, measured by the PLS experiment, to identify the plasma sheet and to study its configuration. We find a pronounced dependence of the count rates on the System III longitude of the spacecraft, with one or two maxima per Jovian rotation, consistent with the generally accepted assumption of a thin plasma sheet in the magnetic equatorial region, which propagates outward as a surface wave generated by the rotation of the tilted magnetic dipole of Jupiter. The inferred effective propagation speed agrees approximately with previous estimates. With Galileo located essentially in the equatorial plane, the plasma sheet can be traced unambiguously to distances of 100RJ and beyond. A new result, made possible by the long duration of the Galileo observations, is that the observed rotational variation of the count rates is subject to a longer-term, ≈5--7 day modulation, indicating changes in the mean location, or the surface-wave amplitude, or the thickness of the plasma sheet, or a combination of these. In at least one instance, only a change of mean location can account for what is observed. The cause of this modulation has not been yet definitively identified. Solar wind effects, dynamics of plasma supply from the Io torus to the plasma sheet, variability of the plasma source due to volcanic activity of Io, and drifting longitudinal asymmetry of the hinging distance for the plasma-sheet surface wave are among the possibilities.¿ 1997 American Geophysical Union