Millet butterflyfish stripe runs from above the eye to the base of the operculum, and there is a large black patch on the caudal peduncle. This fish is endemic to the Hawaiian Islands and the Johnston Atoll, and is found down to depths of at least . Its habitats include coastal fringing reefs, outer reefs and lagoons. The millet butterflyfish is a schooling species found in mid-water that feeds on zooplankton. It also feeds opportunistically on polychaete worms and other small invertebrates on the seabed. Food is drawn into the mouth by rapid suction. It does not seem to be territorial but
Breitensteinia in five to six longitudinal rows on each side, a dorsal profile rising evenly but not steeply from the tip of snout to the origin of dorsal fin and then sloping gently ventrally from there to the caudal peduncle, a horizontal ventral profile to the origin of the anal fin and then rising to the caudal peduncle, a dorsal fin origin nearer to the tip of the snout than caudal flexure, and a truncate caudal fin. "B. hypselurus" has a smooth posterior edge to its dorsal fin spine, relatively tall neural spines on the caudal vertebrae, a shorter caudal peduncle,