We study waveforms of electrostatic solitary waves (ESW) and corresponding electron velocity distribution functions observed by the Geotail spacecraft. When we observe a series of ESW in the plasma sheet boundary layer of the Earth's magnetotail, we find enhanced fluxes of high-energy electrons flowing along the ambient magnetic field. We find good correlation between the propagation direction of the ESW and the direction of the enhanced high-energy electron flux. This supports the model proposed by computer simulations that ESW are formed by the electrons modulated through the bump-on-tail instability. The enhanced electron flux is regarded as the diffused electron beam after the saturation of the instability. From comparison between the variation of waveforms and the velocity of the electron beams, the spatial scales and depths of the ESW potentials and their propagation directions are estimated. We found both tailward and earthward ESW in the plasma sheet boundary layer, even in the deep magnetotail. ¿ 1999 American Geophysical Union