Gap detection was measured as a function of noise bandwidth with constant high-frequency cutoff in both normal-hearing and cochlear-impaired listeners. Band-widening functions were measured in a low-frequency region (0.6-kHz upper cutoff) and a high-frequency region (2.2-kHz upper cutoff). Measures of frequency selectivity were also obtained in the two frequency regions. The results for the normal-hearing listeners indicated that, on a double logarithmic scale, gap thresholds improved more steeply with increasing bandwidth in the higher frequency region than in the lower. However, performance at the narrow bandwidths was independent of frequency region. For the cochlear-impaired listeners, gap thresholds were generally longer than normal in both frequency regions. However, in the higher frequency region, progressively more listeners approached normal values as bandwidth was increased. The elevated gap thresholds were due in part to the higher absolute thresholds and in part to poor ability to process stimulus fluctuation. No correspondence was found between gap detection performance and frequency selectivity.