Voyager spacecraft observations of Z mode waves in Jupiter's magnetosphere are analyzed. The signal appears in wideband spectrograms as an intense narrowband emission separated from the broadband continuum radiation seen at higher frequencies by a deep emission gap. The assumption that the intensity minimum corresponds to the electorn plasma frquency organizes the data very well and provides a consistent interpretation of all spectral features in terms of plasma resonances and cutoffs. Several examples are given which demonstrate that the continuum radiation is composed of both left-hand and right-hand polarized waves with distinct cutoffs observed at the plasma frequency and right-hand cutoff frequency, respectively. A survey of a representaive sample of events reveals that the Z mode peak frequency lies close to the left-hand cutoff frequency, suggesting that the observed characteristics of the emission are the result of wave reflection at the cutoff layer. In a frequency is detected simultaneously with the Z mode. The entire set of observations gives strong support to the linear mode conversion theory of upper hybrid waves to continuum radiation mediated by the Z mode via the Budden radio window mechanism. ¿American Geophysical Union 1990