The effective bandwidths of cat auditory‐nerve fibers were estimated electrophysiologically from the masked thresholds of tones at the nerve fibers’ characteristic frequency. The masking noise was of variable bandwidth but constant total power and was geometrically centered on the characteristic frequency. It was presented at an intensity which, for the narrowest masker bandwidth used, raised the tone threshold by about 20 dB. The point at which the masked tone threshold began to decline as the masker bandwidth was increased provided an estimate of the fiber’s effective bandwidth. A similar method had been used in earlier psychophysical estimates of the cat’s critical band. The auditory‐nerve‐fiber bandwidths were on average 6 dB less than the critical bandwidth at the same frequency. Two of the cats had been tested both psychophysically and electrophysiologically; in these cats the difference was 6.8 dB. Most fibers, moreover, had critical ratios less than that observed psychophysically. The results suggest that the critical band is not a simple correlate of the tuning curves of auditory‐nerve fibers.Subject Classification: [43]65.42, [43]65.58.