AbstractAerial and underwater audiograms for two young female northern fur seals (Callorhinus ursinus) and one young female California sea lion(Zalophus californianus)were obtained with the same procedure and apparatus.Callorhinushears over a larger frequency range and is more sensitive to airborne sounds thanZalophusor any other pinniped thus far tested in the frequency range of 500 Hz to 32 kHz. Sensitivity ofCallorhinusto waterborne pure tones, ranging from 2 to 28 kHz, is equal or superior to all other pinnipeds tested in this same frequency range. LikeZalophus, the upper frequency limit for underwater hearing (as defined by Mastertonet al.1969) inCallorhinusis about one‐half octave lower than the three phocid species thus far tested.Callorhinus'upper frequency limit in air is about 36 kHz and under water it is about 40 kHz. Comparison of air and water audiograms showsCallorhinusis no exception to previous behavioral findings demonstrating that the „pinniped ear” is more suitable for hearing in water than in air. Similar toZalophusandPhoca vitulina, Callorhinusshows an anomalous hearing loss at 4 kHz in air. The basis for this insensitivity to airborne sounds at 4kHz and not at lower or higher frequencies is presumably caused by specialized middle ear mechanisms matching impedance for waterborne sounds. Critical ratio curves forCallorhinusare similarly shaped to ones obtained for humans but are shifted upwards in frequency. Compared to all other marine mammals thus far evaluated, the critical ratios forCallorhinusare the smallest yet rep