We describe in new component of Jupiter's radio spectrum. This component emits in a very narrow bandwith (?40 kHz) near 100 kHz. Its waveform is a very smooth and gradual rise and subsequent fall in intensity over typically two hours. The emission is polarized with left-hand polarization associated with the Jovian northern magnetic hemisphere and righthand with the south. The most interesting feature of the emissionis its deviation from a strict System III rotation period repetition rate. The emission source of this narrow-band component clearly rotates slower by 3--5% than all other forms of Jovian radio emission. propagation considerations coupled with this observed lack of corotation point to a source region near the magnetic equatorial plane at the outer 'edge' (8-9 RJ) of the Io plasma torus.