Sexual dimorphism in otoliths is recorded here for the first time (except for a single report in Cichlidae by Gaemers and Crapono de Crapona, 1986). Within the genus Neobythites, about half of the species exhibit sexual dimorphism, whereas the other half does not. Morphological alteration only occurs in the otoliths of males and is always very much of the same nature. The change is so evident that male otoliths can easily be recognized as such. Otoliths of female fishes remain practically unchanged. Sexual dimorphism in otoliths is also evident from the monotypic genus Spectrunculus, a genus which is also known for its highly sexually dimorphic swimbladder pattern. The function and role of the observed sexual dimorphism in the social life of the fishes is not known. An obvious explanation would be a specific sound receiving ability designed to locate mating partners in the deep sea, where they live. A review of the few fossil otolith records related to both genera not only revealed three new species described here, but also brought to light several likely cases of sexual dimorphism in previously described species hitherto erected as separate species. One species is Neobythites from the Australian Eocene and three other possible cases from the European Tertiary from the genus Hoplobrotula. This implicates that sexual dimorphism of otoliths was already achieved at a relatively early stage in the history of these genera. An unusual case of ontogenetic dimorphism is observed in the genera Spectrunculus and Pycnocraspedum, in which otoliths of juvenile fishes in a way seem to me more ‘specialized’ than those of adults. This observation probably related to the pelagic life style of the juveniles and the more benthic life style of adults.