Kinetic localization of beam-driven Langmuir waves is studied for parameters relevant to the electron foreshock environment. Particle-in-cell simulations of the interaction occurring between energetic electrons and Langmuir waves are performed. It is shown that the nonlinearities in the resonant electrons lead to the formation of packets growing out of the noise. The characteristic spatial scale depends upon the beam velocity and has a weak dependence on the wave amplitude. The mechanism takes place within shorter timescales and for smaller amplitudes than the classic nonlinearities due to bulk electrons and ions. As an example, we apply the results to observations made in the Earth and Jovian foreshocks. ¿ American Geophysical Union 1996