Otophysine fishes (carps and minnows, catfishes, characins, knifefishes) comprise about 8000 species and are the most successful freshwater fish group. They have evolved mammal-like structures for hearing, namely a chain of tiny ossicles transmitting oscillations from the swimbladder, which acts as a tympanum or ear drum, to the inner ear. These Weberian ossicles enable otophysines to detect sound pressure and extend their hearing range to lower sound intensities and to sound frequencies of up to several kHz. They represent the largest group of hearing specialists. Hearing enhancement by the Weberian ossicles is frequency dependent. Removal of the tripus, the largest ossicle, in goldfish decreased hearing sensitivity as measured by the non-invasive auditory evoked potential (AEP) recording technique from 7 dB at 100 Hz up to 33 dB at 2 kHz. Despite their common trait, otophysines reveal a large diversity in hearing sensitivities both between and within orders. This is linked to swimbladder and ossicle morphology, which are in particular diverse in catfishes. Several families (doradids and ariids) possess large, free, unpaired swimbladders, while others (loricariids) have paired tiny swimbladders totally encapsulated in bone. Investigations of 11 species from 6 families showed that large bladders and ossicles as well as higher ossicle numbers improve hearing ability at higher frequencies. Which ecological or social constraints prompted the evolution of the Weberian apparatus in otophysine species? The similarity of auditory sensitivity between vocalizing and non-vocalizing species indicates that the Weberian apparatus evolved prior or mostly independently from the development of sound-generating mechanisms. Predator and/or prey detection in quiet freshwater habitats were probably the major selective pressures behind this hearing specialization.