The development of the osteocranium of the suckermouth armored catfish Ancistrus cf. triradiatus is described based on specimens ranging from prehatching stages to juvenile stages where the osteocranium is more or less fully formed. The first bony elements that arise are the opercle, jaws, and lateralmost branchiostegal rays, as well as the basioccipital and parasphenoid in the skull floor. The supracleithrum and the membranous and perichondral pterotic components form one large, double-layered skull bone during ontogeny, without clear evidence of the involvement of a supratemporal. The Baudelot's ligament ossifies from two sides, i.e., from the basioccipital medially and the supracleithrum laterally. The lower jaw consists of a dentary, mentomeckelian, and angulo-articular, which all soon fuse. The parurohyal, formed by the fusion of a ventral sesamoid bone and a dorsal cartilage element associated with the first basibranchial, is pierced by a vein, unlike in some other siluriforms. The interhyal cartilage disappears during ontogeny; medially of it, a small sesamoid bone appears in a ligament. The largest, canal-bearing cheek plate is not homologous to the interopercle. The results of the present research, with emphasis on bone formations and homologies, are compared with studies on related catfishes. J. Morphol., 2007. © 2007 Wiley-Liss, Inc.