Horny projections arising from single cells, or unculi, of ostariophysan fishes are described and discussed by several workers, but they occur in many more families and genera than previously reported and their status as a key adaptive feature has not been recognized. Unculi are related morphologically to the relatively well‐known multicellular horny tubercles (including breeding tubercles) of Ostariophysi and other fishes, but differ from them in anatomical distribution and function. Unculi are especially prominent features of portions of the epidermis in Cyprinoidei (carps, loaches, and their allies) and Siluroidei (catfishes); they also occur in Characoidei (characins) and Chanoidei (gonorynchs) but have not been found in any non‐ostariophysan fishes. Unculi evidently are functionally significant on the lips and other mouth parts (especially in groups lacking jaw teeth), on the ventral surface of the paired fins in many bottom‐dwellers, and on several neomorphic features including epidermal plaques and tubercles in various catfishes, the thoracic adhesive disc of sisorid catfishes, and the mental adhesive disc of the cyprinid genusGarra. In some sisorid and akysid catfishes virtually the entire external body surface is covered by unculiferous plaques or tubercles. Possible functions of unculi include: mechanical protection of the skin, rasping, adhesion, and hydrodynamic effects. Unculi seem to have been especially important in the diversification of cyprinoid feeding habits and in the adaptation of bottom‐dwelling cyprinoids and siluroids to swift‐water habitats, especiall