This genus is characterized by two synapomorphies. One of these is modified jaw muscles. The other, more obvious trait, is only found in juveniles and subadults, in which the upper and lower rays of the caudal fin are extended into long filaments. "B. vaillantii" is the sister species to all other "Brachyplatystoma", marked by differences in the skull, jaw, and other bones. The subgenus "Malacobagrus" is characterized by their derived lateral line, Weberian apparatus, a subquadrangular gill cover (as opposed to triangular in other species), and pectoral spine.