betta (bĕt´ə) or fighting fish, small, freshwater fish of the genus Betta, found in Thailand and the Malay Peninsula. Best known is the Siamese fighting fish, Betta splendens. Mature males of this species are about 2 in. (5 cm) long. In its native waters B. splendens is drab with small fins, but several centuries of breeding have produced multicolored varieties with extremely enlarged decorative fins, highly prized as aquarium fishes. Males of this species are extremely aggressive, and in Thailand they are used in fighting contests lasting as long as six hours, with spectators betting on the outcome. Bettas thrive in shallow, sunlit areas with soft or sandy bottoms. Males secrete a mucous, with which they build bubble nests. After the female of a pair lays her eggs, both members transfer them to the nest, which is then guarded by the male. Several hundred young hatch out in 24 to 30 days. Like its relatives the gourami and the climbing perch, the betta is equipped to breathe air as well as water and must surface from time to time. It is classified in the phylum Chordata, subphylum Vertebrata, class Actinopterygii, order Perciformes, family Osphronemidae.