Abstract
Astyanax guianensis is redescribed based on the holotype, paratypes, and additional specimens from the rio Essequibo in Guyana, rio Orinoco in Venezuela and from several localities in the Amazon river basin in Brazil, Colombia and Bolivia. Astyanax guianensis is diagnosed by having five to 10 maxillary teeth, 31 to 35 pored lateral-line scales, 21 to 25 branched anal-fin rays, and a dark vertical humeral blotch followed by a clear area and then by a dark longitudinal stripe, ending before the caudal-fin rays. In addition, comments on the importance of revisionary studies on the species of the Characidae are provided.