A theoretical and experimental study is presented of ac losses in the mixed state of type-II superconductors when viscous damping dominates flux line motion. The theory describes the ac response of a cylindrical superconductor in the mixed state in a dc magnetic field H and a superposed parallel ac field of arbitrary frequency. The complex ac permeability is found to be related to the flux-flow resistivity rho/sub f/(B), the differential permeability ..mu..' (B) = dB/dH, the frequency, and the sample radius. Measurements were made of the ac permeability in the frequency range 45--1000 Hz of two NbTa alloys which were prepared to minimize bulk-pinning and surface losses. Values of rho/sub f/(B) and ..mu..' (B) derived from the measurements agree qualitatively with the model, but rho/sub f/(B) shows a weak frequency dependence that may arise from the presence of remnant nonviscous losses. (AIP)