Reported are measurements on the high‐frequency fields generated by a relativistic electron beam plasma interaction in the BP II experiment. Measurements are done at fixed positions as well as integrated over the length of the experiment. At fixed positions time‐resolved spectroscopy on hydrogen and helium lines is applied. The integrated measurements are performed with a magnetic energy analyzer, which measures the energy distribution of the relativistic electron beam at the end of the experiment. Spectroscopic measurements of the dynamic Stark effect and forbidden transitions lead to hf fields with amplitudes in the order of 1 MV/m. The interpretation of the beam energy spectra suggests localized fields with field strengths as great as 108V/m.