The common thresher, Alopias vulpinus, is a species of thresher shark, family Alopiidae, found in tropical and temperate waters worldwide. The largest member of its family, this species may attain a length of 6 m (18 ft), half of which consists of the long upper lobe of its tail fin. The common thresher is often confused with the similar-looking pelagic thresher (Alopias pelagicus), from which it can be distinguished by the white band over its pectoral fins.
This shark is an active, fast-swimming predator that is capable of maintaining a higher body temperature than the surrounding sea water. Its long tail is used in a whip-like fashion to stun its prey, mostly small schooling forage fish. Like other mackerel sharks, it has an ovoviviparous mode of reproduction and gives birth to live young. The common thresher is commercially important in many parts of the world: the meat and fins are sold for human consumption, the skin is made into leather, and the liver oil is processed for vitamins. This species is also valued by recreational anglers as it is an exceptionally strong fighter on hook-and-line. As a result of increasing exploitation, the common thresher was assessed as Vulnerable by the International Union for Conservation of Nature (IUCN) in 2007.