The wideband plasma wave data acquired by Voyagers 1 and 2 in Jupiter's magnetosphere frequently contain a narrow-band emission of variable intensity whose upper frequency is below the lower-frequency cutoff of the continuum radiation. We have found examples in over 400 wideband frames taken outside the plasma sheet between 20 and 90 RJ from the planet in the dayside and nightside magnetosphere. The narrow-band emissions do not appear to be electrostatic waves. If they are electromagnetic, they can be identified by relating their frequencies to the fundamental frequencies of the ambient plasma and the solutions to the cold plasma dispersion relation. Since the electron plasma frequency was not measured at the rate and accuracy we need, we attempt to infer it by testing two hypotheses about the lower-frequency cutoff of the continuum. By analogy with terrestrial observations, we assume that the continuum cutoff is either at the plasma frequency or the right-hand cutoff frequency at which the right-hand polarized extraordinary mode cuts off. The measured electron cyclotron frequency then enables us to determine the wave mode of the narrow-band emissions. More consistent results followed from assuming the continuum cut off at the right-hand frequency. Under this assumption the narrow-band emission falls between the left-hand cutoff frequency and upper hybrid frequencies, which identifies the waves as the slow branch of the X mode, or the so called Z mode. This also implies that the continuum is comprised or R-X mode waves where the Z mode is observed as a separate band, since the L-O mode would fill the gap between the Z mode and continuum bands. As Voyager 1 approached the plasma sheet on March 8, 1979, the Z mode intensified and then disappeared on plasma sheet entry. We interpret this as evidence of local Z mode generation. ¿American Geophysical Union 1987