SUMMARY The structural and ultrastructural characteristics of the olfactory epithelium in four species of cyprinids are described: Barbopsis devecchii Di Caporiacco, Phreatichthys andruzzii Vinciguerra, Caecobarbus geertsi Boulenger, all African hypogean fish, are compared with the common European species Barbus barbus plebejus Valenciennes. At all levels of investigation a substantial morphological uniformity has been verified among the four species examined, while no compensation, qualitative or quantitative, seems to have accompanied the adaptation to the subterranean life in the three hypogean species, in relation to the partial or total regression of the visual system. Fine details are particularly given about the two types of receptors, with cilia and with microvilli, that were observed and about the peculiar supporting cells which show an intense secretory activity. In two species of cyprinids, «rodlet cells», whose nature is still an open question, have been recognized.