Large intensities of low-energy protons were observed deep within the Jovian magnetosphere with the plamsa instrumentation on Pioneer 10 during the encounter of this space probe with Jupiter. The energy range of the electrostatic analyzer was 108 eV to 4.80 keV during encounter. Inside the flux tubes of the Galilean moon lo is a 'plasmasphere' of protons with relatively high densities, 50--100 cm-3, extending toward the planet to at least 2.8 RJ. The characteristic thermal energies of these protons are about 100 eV. The flux tubes of lo are positioned on a severe decrease of these densities with increasing Jovicentric radial distance-a plasmapause. The relationship of Io to this plasmapause is quite likely to be fundamental to the lo modulation of decametric radio emissions. The proton densities in the vicinity lo are also sufficiently high to limit the azimuthal extent of the partial torus of hydrogen gas from lo by the mechanism of charge exchange. At greater distances, beyond the plasmapause, is found a great torus of plasma encircling Jupiter with densities in the range 10--15 cm-3 and thermal energies about 400 eV. The moon Europa is embedded in this torus, or 'ring current.' This ring current is extended into a thin plasma disc at greater radial distances. At 15 RJ the thickness of this disc is only 2 RJ, and proton densities are about 1 cm3. The source of these low-energy protons is believed to be the Jovian ionosphere. The mechanisms for the formation of the Jovian plasmasphere, ring current, and plasma disc must differ substantially from those dominantly participating in the terrestrial magnetosphere.