We present the results of the Geotail waveform observation on the electrostatic plasma waves with their frequency close to the local electron plasma frequency in the tail lobe. Those waves, which we call electron plasma waves in the present paper, are strongly modulated in their amplitudes and are very similar to the modulated Langmuir waves observed by previous spacecraft in the various regions. From the detailed analyses, we conclude that they include the Langmuir waves and 5 types of the Electron Cyclotron Harmonic (ECH) waves. Finally, we introduce an interesting event that the observed waves change from the ECH waves to the Langmuir waves in a short period within one second. ¿ 1997 American Geophysical Union