The Io plasma torus apparently has a much longer lifetime than anticipated on the basis of simple theoretical stability analyses. This quandary is particularly evident at the outer edge of the torus, the plasma ramp, where the steep radial gradient in plasma content indicates the presence of some confining influence. Previous attempts to explain this feature have focused on the possibility that high-energy particles could impound the colder, Iogenic plasma, but these studies have proved inconclusive. We consider an alternative mechanism whereby the development of an unstable perturbation is interrupted by the observed shear in the rotational velocity. An example in simplified geometry demonstrates how the relative azimuthal displacement of radially adjacent perturbations might eliminate their coherency and impose a finite saturation amplitude. Fully nonlinear numerical simulations using the Rice convection model at Jupiter produce analogous results, suggesting that perturbations are suppressed where the shear is strong. The crucial parameter is the ratio of the classic, linear growth rate to a rate that characterizes the velocity shear. The electric fields produced by unstable perturbations farther out are effectively shielded from the shear region. We suggest that this effect helps impound the plasma torus and is at least partly responsible for producing the ramp. ¿ 1998 American Geophysical Union