Experimental measurements of the electron distribution function are used to study the stability of plasma in the auroral ionosphere. Measurements were taken at altitudes of 200-250 km and cover the 25-eV to 15-keV energy range. The observed plasma is shown to be stable to electrostatic waves that propagate exactly parallel or perpendicular to the magnetic field. The measured distribution function has a secondary peak, but this peak is either too small or too limited in extent to drive parallel or perpendicular instabilities. A preliminary study of oblique electrostatic waves during one time interval indicates that the upper hybrid mode is stable for all propagation directions at the rocket location but becomes weakly unstable several hundred kilometers higher where the background plasma density drops below 3¿104el/cm3.