Population samples of algae-grazing, mountain stream characins of the genus Saccodon of Panama and northwestern South America are reported with up to four sharply differentiated dental morphs. The morphs differ strikingly in the form and numbers of functional teeth and of preformed replacement teeth. Recognition of the polymorphic nature of these variations necessitates new concepts of taxa at species and generic levels. Two species with similar polymorphism are involved, Saccodon dariensis from Panamá and Colombia, and Saccodon wagneri from the Guayas basin on the Pacific coast of Ecuador. A total of rive morphs have been discovered, three of which are homologous in the two species.