Sternopygus is a genus of weakly electric knifefish in the order Gymnotiformes, the same South and Central American lineage that includes the electric eel, with atlas records from Brazil, Colombia, Venezuela, Ecuador, French Guiana, Guyana, Suriname, and Panama. The 12 species covered here show an unusually wide size spread, from about 1.5 in up to 55.5 in, reflecting a mix of small stream-dwelling forms and much larger, eel-like species from slow, often turbid rivers and floodplains. Like other gymnotiforms, Sternopygus lack a dorsal fin and swim mainly by undulating a long ventral ribbon fin, generating a continuous weak electric field from a specialized organ used for orientation and communication in dark or murky water.
These are largely nocturnal, secretive fish rarely encountered outside scientific collecting or the specialist electric-fish hobby; Sternopygus macrurus, the longtail knifefish, and Sternopygus aequilabiatus, the rattail knifefish, are the two species with widely used common names in the atlas. Half of the 12 species are rated Least Concern and the other half remain Not Evaluated, consistent with a genus more often studied for its electrophysiology than kept for aquarium display.