The plasma wave instruments on the Voyager spacecraft have detected intense electromagnetic radiation within the lobes of Jupiter's magnetic tail down to the lowest frequency of the detector (10 Hz). During a yaw maneuver performed by Voyager 1 in the lobe of the Jovian magnetotail, a modulation appeared in the amplitudes of waves detected in the 10--, 17.8-- and 31.1--Hz channels of the plasma wave analyzer, well below the local electron cyclotron frequency of 260 Hz. The lowest amplitudes occurred when the antenna axis was most nearly parallel to the magnetic field. Wave amplitudes in the 56.2-Hz and higher frequency channels remained nearly constant during the maneuver. From the cold-plasma theory of electromagnetic waves, one can conclude that the plasma frequency was between the 56.2-- and 31.1--Hz channels where the parallel-polarized component of the spectrum cuts off. This implies a taillobe density between 3.2¿10-5 and 1.5¿10-5 cm-3. The left-hand cutoff frequency would then be below 10 Hz, consistent with either the Z mode (L, X) or whistlers (R mode) in the modulated channels. However, locally generated whistler mode waves at these frequencies would require electrons with resonant energies greater than 25 MeV, which are unlikely to exist in significant densities. ¿American Geophysical Union 1987