A recent effort to determine the plasma density by using the the low-frequency cutoff of trapped continuum radiation in the vicinity of the Jovian plasma sheet has revealed the existence of sharply defined density structures in the plasma sheet. These structures typically have a plasma density which is relatively constant, but of order 50% greater or less than that of the surrounding plasma. The transitions from low to high density occur on time scales of about ten seconds, which correspond to spatial dimensions of the order of a few ion Larmor radii. Voyager requires time intervals of less than a minute to more than five minutes to traverse the structures themselves, corresponding to size scales from a fraction of a Jovian radius to more than a Jovian radius. The existence of such structures in the Jovian magnetosphere raises obvious questions as to their three-dimensional nature as well as their origin. We suggest that the structures might represent flux tubes of varying plasma content, but on the basis of single-point measurements, we do not have the observations necessary to describe the three-dimensional form of the structures. A Fourier transform of the density variations yields a featureless spectrum which displays an inverse frequency dependence. This spectrum is consistent with the square wave-like form of the density variations observed. The observations of flux tubes in the middle magnetosphere of widely varying plasma content may be the result of a centrifugally-driven plasma transport mechanism acting inwards of the point of observation. We conclude that the structures observed are not consistent with an eddy-diffusion model. ¿ American Geophysical Union 1992