Fine structures observed in the auroral kilometric radiation (AKR) and in the Jovian decametric radiation (DAM) show remarkable similarities to fine structures in discrete VLF emissions. The feedback model of Helliwell for discreate VLF emissions is modified to apply to ARK and DAM. An important constraint on the model is that the frequency drift must be close to that characteristic of a propagating electron radiating at its cyclotron frequency. For DAM the model fits satisfactorily with the observational data. For AKR it seems necessary to assume that the basic fine structure is unresolved and that the reported fine structures are envelopes of these basic structures drifting at a rate associated with the exciting agency, which is presumed to be a localized parallel electric field. It is argued that these basic fine structures may be drifting rapidly at a rate proportional to the parallel electric field (provided it is > 0.1 mV m-1) and that their bandwidth, determined by the relativistic velocity spread, should be less than ≂ 1 kHz. Triggering of AKR by type III bursts can be explained naturally in terms of the model.