An alternative explanation of the fine frequency structure (≤1 kHz) associated with auroral kilometric radiation (AKR) is presented. It is based on a realistic model of the electron distribution function inside the AKR source region. The resulting cyclotron maser instability growth rate is shown to possess an extremely narrow bandwidth, of the order Δ&ohgr;/&ohgr;~10-3, which may naturally account for the AKR fine frequency structure. The results differ from many previous calculations and simulations which relied on simpler electron distribution function models such as a loss-cone, ring-beam or DGH distribution. ¿ 1998 American Geophysical Union