When a flexible-walled cavity resonator undergoes flow-induced (self-excited) oscillations in the presence of a grazing flow, its behavior can be fundamentally different from that of the same resonator undergoing forced oscillations. One difference is that the bandwidth of oscillations may be much narrower in the self-excited case: Q’s measured for this case are typically an order of magnitude higher than Q’s calculated using a simple lumped-element model. An explanation of this phenomenon is given, using the root-locus method of Cremer and Ising along with an approximate formulation for the impedance of the resonator opening. The impedance formulation incorporates ‘‘negative resistance’’ associated with instability of the mean flow, and nonlinear contributions to the resistance, which serve to limit the amplitude of oscillations. [Work supported by the William E. Leonhard Endowment to the Pennsylvania State University.]