Maximum electric fields of Langmuir waves at planetary foreshocks are estimated from the threshold for electrostatic decay, assuming it saturate, beam-driven growth, and incorporating heliospheric variation of plasma density and temperature. Comparison with spacecraft observations yields good quantitative agreement. Observations in type III radio sources are also in accord with this interpretation. A single mechanism can thus account for the highest fields of beam driven waves in both contexts. ¿ 1995 American Geophysical Union