Anatomical and histological parameters of the bulla tympanica of the Amazon river dolphin ( Inia geoffrensis ) have been compared with those of two Black Sea species, bottle-nosed dolphin ( Tursiops truncatus ) and common dolphin ( Delphinus delphis ), as related to their ecological characteristics. The ecological‐morphological approach has made it possible to establish that the morphofunctional adaptations aimed at optimizing acoustic sensitivity in the aquatic environment are the main factors of the evolution of the organ of hearing in dolphins. The material for histological studies was fixed with 10% neutral formalin and Wittmaack’s fixative. Serial celloidin sections 10‐15 µ m in width were stained with hematoxylin‐eosin or by the methods of Mallory and Kulchitsky or impregnated with silver nitrate. A total of six bullae tympanicae of T. truncatus , six ones of D. delphis , and two ones of I. geoffrensis were studied.