  We present the results of a study of vortex arrangements in the peak-effect
regime of 2H-NbSe_2 by scanning tunneling microscopy. By slowly increasing the
temperature in a constant magnetic field, we observed a sharp transition from
collective vortex motion to positional fluctuations of individual vortices at
the temperature which coincides with the onset of the peak effect in
ac-susceptibility. We conclude that the peak effect is a disorder driven
transition, with the pinning energy winning from the elastic energy.
