The responses of single auditory‐nerve fibers in anesthetized cats to tonal stimuli were recorded with microelectrodes. Sound‐pressure levels were measured at the eardrum. After correcting for the effects of the middle ear, we were able to compare sensitivities of fibers with different characteristic frequencies. It is difficult to find a single response criterion that will serve as a satisfactory measure of sensitivity for all fibers. Data are presented to show the results of using several sensitivity measures for fibers with different rates of spontaneous activity and characteristic frequencies. The effects of previous stimulation and choice of stimulus parameters on the responses of these fibers are discussed. [Work supported in part by the Joint Services Electronics Program, by the National Science Foundation, by the National Institutes of Health, U. S. Department of Health, Education, and Welfare, and by the National Aeronautics and Space Administration.]