Diagnosis: This species can be distinguished from all congeners by the presence of a single well-defined, round to horizontally oval humeral spot, similar to the one found in species of the Astyanax bimaculatus (Linnaeus) complex (vs. humeral spot absent, double, or variously developed, never round to horizontally oval). It can also be distinguished from its congeners, except H. reticulatus Ellis, by the combination of a reticulate dark pigmentation pattern, a broad, horizontally-elongate caudal-peduncle blotch, a narrow dark stripe extending along the central caudal-fin rays, 5 or 6 horizontal scale rows between the dorsal-fin origin and the lateral line, 4 or 5 horizontal scale rows between the lateral line and the pelvic-fin origin, and 15-20 branched anal-fin rays. Hyphessobrycon langeanii can be distinguished from H. reticulatus by the possession of a conspicuous, dark round to oval humeral spot (vs. relatively faint, vertically-elongate humeral spot), a horizontally-elongate caudal peduncle blotch (vs. vertically-elongate caudal peduncle blotch), the midlateral dark stripe relatively wide and faint (vs. midlateral dark stripe extremely narrow and well-defined), and infraorbitals 3 and 4 co-ossified (vs. separated) (Ref. 52338).