Astroblepidae or “climbing catfishes” encompass a single genus of species living in high altitude rivers in the Andes of South America. They are characterized by a specialized head morphology closely resembling their better known, widely radiated sister family Loricariidae, or armored suckermouth catfishes. Existent data show that even though both families share important traits, there are some striking differences as well. Albeit poorly known, Astroblepus species possess a duplicated gill opening, and have the ability to climb vertical rocks or waterfalls. In this study, morphological and kinematic data are combined to yield insights into the functions of the mobile elements of the astroblepid head, and to compare head morphology and biomechanics with those of Loricariidae. We found that, even though there is substantial similarity in head structure of both families, there are major differences in functionally important structures. These include a different lower lip muscle configuration, an alternative oral valve system, and an incurrent gill opening only found in astroblepids. Kinematic analyses confirm that the astroblepid suckermouth, freed from its inhalatory function, offers advantages for climbing in the high-altitude environment, and is used alternately with the extremely mobile pelvic girdle, in a crawling, nonundulatory motion. J. Morphol. 274:1164–1179, 2013. © 2013 Wiley Periodicals, Inc.