Accurate measurements of the harmonics of foreshock Langmuir waves have been made with the S/WAVES experiment on STEREO. The amplitude of the harmonics relative to the fundamental is quite small, on the order of 1%. This ratio varies widely, showing that it cannot be due to nonlinearities either in the circuit or in the antenna sheath, as these effects would be expected to produce amplitudes which are definite functions of the fundamental amplitude. Neither are the observed harmonics due to an intrinsic nonlinearity of Langmuir waves. Trapping of electrons in the Langmuir wave potentials seems to be what is happening. The waveforms are shown to be consistent with this interpretation. As there is no reason to expect that a propagating Langmuir wave would trap electrons, it appears that they are probably trapped as the Langmuir wave grows. The harmonics may give some indication of the age of the Langmuir waves, therefore.