We present a derivation of the quasi-thermal electrostatic fluctuation power spectrum in a multi-Maxwellian plasma and show sample calculated spectra. We then apply this theory, which has been successfully applied in other regions of space, to spectra from two AMPTE IRM passes through the duskside plasmasphere. We show that the plasma line that is often seen in this region is usually quasi-thermal in origin. We obtain a refined estimate of the plasma frequency and infer a cold electron temperature which is consistent with a factor of 2 with both models and previous measurements by other techniques, but closer investigation reveals that details of the plasma line cannot be explained with the usual two isotropic Maxwellian model. ¿ American Geophysical Union 1994