Frequency difference limens (DLs) were obtained for frequencies (F) from 1 to 140 kHz for the bottlenosed dolphin, T u r s i o p s t r u n c a t u s (Montagu), and from 1 to 8 kHz for two human subjects tested underwater. Discriminations were required between constant‐frequency (pure‐tone) signals and frequency‐modulated signals, using a successive discrimination procedure. Relative DLs (DL/F) for the dolphin generally ranged from 0.002 to 0.004 between 2 and 53 kHz, and never exceeded 0.008 through to 130 kHz; at 1 and 140 kHz, relative DLs increased to 0.014. No responses were obtainable at 150 kHz. These findings demonstrate excellent frequency discrimination throughout the audible spectrum (above 1 kHz) of T u r s i o p s and support electrophysiological evidence of highly sensitive frequency detection mechanisms. Results for the humans showed smaller DLs than the dolphin at 1 kHz, approximately equal DLs at 2 kHz, and progressively larger DLs at 4 and 8 kHz. The human underwater thresholds were generally consistent with typical human in‐air measurements of frequency DL. Subject Classification: 65.54, 65.22; 80.50.