We reexamine the stability of low-frequency electrostatic waves with k⋅B=0 at the plasmapause, using an extension of the procedure of Richmond (1973), an approch that is based on computing individual particle motions and includes the line-tying effect of the ionosphere. We derive expressions for the complex frequency as a function of wave number. The effect of the polarization (inertial) current accompanying the wave is included in an approximate way. The instability is caused by the sharp change in plasma pressure at the plasmapause, but its growth rate is limited by ionospheric conductivity and, for very short wavelengths, by the inertia of the magnetospheric particles. The growth rate generally increases with decreasing ripple wavelength, until that wavelength becomes small compared to the thickness of the plasmapause. The ring current (hot and warm plasma) particles suppress wave growth significantly only for long wavelengths. According to our linear analysis, an interchange ripple with wavelength ≲2000 km that forms on a sharp plasmapause (≲0.1 RE) should grow by >10 e folds as it transverses the nightside of the Earth. This result is consistent with Richard's (1973) conclusion that the interchange instability limits the thickness of the plasmapause on the nightside of the Earth. ¿American Geophysical Union 1990