Aerial low-frequency (100–6400 Hz) hearing thresholds were obtained for one California sea lion (Zalophus californianus), one harbor seal (Phoca vitulina), and one northern elephant seal (Mirounga angustirostris). Underwater thresholds over a similar frequency range (75–6300 or 6400 Hz) were obtained for these three animals in addition to another California sea lion. Such data are critical, not only for understanding mechanisms about amphibious hearing and relating them to pinniped ecology and evolution, but also for identifying species at risk to man-made noise in the marine environment. Under water, the elephant seal was most sensitive, followed by the harbor seal and the sea lions. In air, the harbor seal was most sensitive, followed by the older of the two sea lions and the elephant seal. The following trends emerged from comparisons of each subject’s aerial and underwater thresholds: (a) the sea lion (although possessing some aquatic modifications) is adapted to hear best in air; (b) the harbor seal hears almost equally well in air and under water; and (c) the elephant seal’s auditory system is adapted for underwater functioning at the expense of aerial hearing sensitivity. These differences became evident only when aerial and underwater thresholds were compared with respect to sound pressure rather than intensity. When such biologically relevant comparisons are made, differences in auditory sensitivity can be shown to relate directly to ecology and life history. © 1998 Acoustical Society of America.