Dealfish, any of several slender marine fishes that belong to the genus Trachipterus (family Trachipteridae, order Lampridiformes), a subgroup of the ribbonfish. The dealfish inhabits the middle waters, probably not below 400 m (1,300 feet), and is characterized by a long, laterally compressed body, short head, narrow mouth, and feeble dentition. The dorsal fin extends the length of the back, with the anal fin absent and the caudal (tail) fin, when present, much-reduced; the pectoral fins are small. In the young dealfish some of the fin rays (hornlike projections of the fin skeleton) are prolonged in an extraordinary way.
Dealfishes, which may reach a length of 2.5 m (8.2 feet), are distributed worldwide. The most common species is the northern dealfish, T. arcticus.