Summary: A non-inertial (zero Taylor number) viscoelastic instability is discovered for Taylor-Couette flow of dilute polymer solutions. A linear analysis of the inertialess flow of an Oldroyd-B fluid (using both approximate Galerkin analysis and numerical solution of the relevant small-gap eigenvalue problem) show the growth of an overstable (oscillating) mode when the Deborah number exceeds \(f(S)\epsilon^{- 1/2}\), where \(\epsilon\) is the ratio of the gap to the inner cylinder radius, and f(S) is a function of the ratio of solvent to polymer contributions to the solution viscosity. Experiments with a solution of 1000 p.p.m. high-molecular-weight polyisobutylene in a viscous solvent show an onset of secondary toroidal cells when the Deborah number De reaches 20, for \(\epsilon\) of 0.14, and a Taylor number of \(10^{-6}\), in excellent agreement with the theoretical value of 21. The critical De was observed to increase as \(\epsilon\) decreases, in agreement with the theory. At low times after onset of the instability, the cells become small in wavelength compared to those that occur in the inertial instability, again in agreement with our linear analysis. For this fluid, a similar instability occurs in cone-and-plate flow, as reported earlier. The driving force for these instabilities is the interaction between a velocity fluctuation and the first normal stress difference in the base state. Instabilities of the kind that we report here are likely to occur in many rotational shearing flows of viscoelastic fluids.