  Vortex rings self-propelling in superfluid 4He are shown to be driven
unstable by a toroidal normal fluid flow. This instability has qualitative
similarities with the Donnelly-Glaberson instability of Kelvin waves on a
vortex filament driven by the normal fluid flow along the vortex filament. The
growth rate of the present instability is found to be independent of the radius
of the vortex ring.
