Leucos is a genus of fishes in the family Cyprinidae, from Southern Europe. They are roaches close to the genus Rutilus, and were only recently taxonomically distiguished from that genus. Molecular data suggest that Leucos diverged from Rutilus more than five million years ago, probably during the Messinian salinity crisis. The species of Leucos are typically of small size and they all live in still waters. They differ from Rutilus by the lack of spinous tubercles on scales and head in reproductive males, and also in the pharyngeal teeth formula. The sister group to Leucos is the monotypic Sarmarutilus.