Difference limens (DLs) for standard tonal durations (T) of 0.3, o.6 and 1.2 sec were determined underwater for a bottlenose dolphin, Tursiops truncatus, and for a human subject underwater, and in air. For the dolphin, the temporal stimuli were either 9 – or 25 kHz pure tone. A response on a paddle to the animal’s left was reinforced following a standard duration signal, while a response on a right – hand paddle was reinforced following a longer duration signal, DLs being obtained for one standard at a time. The relative DLs (DL/T) generally remained between 0.06 and 0.08 for all three standards of each tonal frequency. Procedures for the human subject were similar to those for the dolphin, except that the temporal stimuli, in water and in air, were 1 kHz pure tones. No appreciable difference was found between the human in – water and in – air relative DLs, but their values were generally more than twice as high as those for the bottle nose dolphin. It was noted that temporal duration information could be useful to the echolocating bottlenose dolphin for the estimation of target distance.