  We report observations of a radio burst that occurred on the flare star AD
Leonis over a frequency range of 1120-1620 MHz (lambda ~18-27 cm). These
observations, made by the 305m telescope of the Arecibo Observatory, are unique
in providing the highest time resolution (1 ms) and broadest spectral coverage
(Delta nu/nu =0.36) of a stellar radio burst yet obtained. The burst was
observed on 2005 April 9. It produced a peak flux density of ~500 mJy and it
was essentially 100% right circularly polarized. The dynamic spectrum shows a
rich variety of structure: patchy emission, diffuse bands, and narrowband,
fast-drift striae. Focusing our attention on the fast-drift striae, we consider
the possible role of dispersion and find that it requires rather special
conditions in the source to be a significant factor. We suggest that the
emission may be due to the cyclotron maser instability, a mechanism known to
occur in planetary magnetospheres. We briefly explore implications of this
possibility.
