Thayeria boehlkei, the blackline penguinfish, is a species of freshwater ray-finned fish belonging to the family Acestrorhamphidae, the American characins. This species is endemic to the Amazon River basin and Araguaia River, in Peru and Brazil respectively. The species is popular with aquarium hobbyists, and is traded under a variety of common names including blackline penguinfish, blackline thayeria, hockey-stick tetra, penguin fish and penguin tetra.