  In this paper I analyze the onset of Rayleigh-Taylor instability between two
linear viscoelastic fluids assuming that the perturbations at the interface are
small. In the first half, the paper analyzes a stratified viscoelastic fluid in
which I prove that the perturbations rise or fall exponentially without
oscillating. The second half of the paper examines the effect of electric and
magnetic fields on viscoelastic fluids. I show that it is possible to choose
electric or magnetic field gradient such that the effective acceleration due to
gravity is zero. If a heavy Newtonian fluid rests on top of a lighter Newtonian
fluid such a choice of field gradient would have rendered the arrangement
stable. If the fluids are viscoelastic, I show that a similar arrangement is
unstable.
