Evidence is given for electron holes in the upward current auroral kilometric radiation (AKR) source region. Isolated parallel electric field structures of tripolar polarity are seen in FAST recordings, being interpreted in terms of trains of nested ion and electron holes such as shown in numerical simulations by Goldman et al. (2003). They are created by beam plasma interaction via the kinetic two-stream instability upstream of a strong double layer, giving rise to broadband emission spectra below the ion plasma frequency. Field amplitudes reach ~1 V m-1 peak-to-peak and appear to modulate both the particle energy fluxes. The presence of electron holes is responsible for the fine structure of AKR emissions.