Though there is indirect evidence to indicate that various species of seals are capable of some form of echolocation, all previous attempts at an experimental demonstration of sonar capacities have met with negative results. I have obtained the following circumstantial evidence that Harbour seals can echolocate: (1) During three breeding seasons I have observed apparently healthy blind adult harbour seals including females which have successfully raised pups. (2) When visual cues are reduced, harbour seals make click vocalizations similar to those of animals which are known to echolocate. (3) They are able to find live fish in total darkness, producing clicks while doing so. (4) One seal trained to retrieve a ring which he first had to find in a 10‐m‐diam tank performed as well in the dark as in daylight, but clicked only when he could not use vision. I obtained a more rigorous demonstration of Harbour seal sonar when I was able to train an animal to discriminate between two visually identical rings which differed in acoustic impedance. When the acoustical properties of the rings were made equal, the animal was no longer able to distinguish between them. Very faint single or doublet clicks were recorded during the animal's performance, and it is suggested that quiet distinct signals may have been appropriate in an enclosed tank when the objects to be discriminated were suspended close to the enclosure's walls. The reasons why the training procedures used in this demonstration were successful are discussed, and the limitations of the conclusions which can be drawn from such results are outlined.

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