The critical ratios of two ringed seals were estimated by determining pure-tone thresholds (yes–no, forced choice) in the presence of various noise bands. Data obtaind from both seals were pooled. The critical ratios (using spectrum noise levels of −30 and −40 dB re 1 μbar) were 30±5.4, 32±3.8, 34±2.8, and 35±4.5 dB at 4, 8, 16, and 32 kHz, repectively. These values are consistent with findings of other mammalian species. In the open ocean, meteorological and man-made noises below 100 kHz may mask the underwater hearing of marine mammals. As there is less than 10-dB variation in the noise (spectrum level) above 100 kHz, an echolocation system operating in this range would be less subject to varying amounts of masking. Subject Classification: 80.50; 65.22, 65.58.