We have analyzed mass-resolved ion, electron, and plasma wave data obtained from several low-latitude boundary layer (LLBL) crossings by the AMPTE CCE satellite. The data clearly separate the LLBL from the adjacent magnetosheath and magnetosphere. The available plasma wave data were limited to high frequencies; therefore we focused our attention on wave-particle interactions involving electrons. We found that electron beams are present in the LLBL during southward interplanetary magnetic field, along with simultaneous enhancement of electrostatic waves with parallel polarization. Linear theory analysis shows that for plasma, conditions in the LLBL, electron beams are unstable to electrostatic waves that propagate parallel to the local magnetic field, in agreement with observations. A numerical simulation study of the beam-plasma interaction in the LLBL has also been undertaken to examine the saturation mechanism. The simulations show that the beam instability saturates by thermalization of the beam but that a beamlike structure can still remain in the electron distribution for certain initial parameters. The results suggest that peaks in the electron velocity distribution function may be found in the LLBL away from the beam source region. ¿ American Geophysical Union 1992