Astyanax argyrimarginatus sp. n. is described from the Araguaia River basin, Brazil. The new species may be diagnosed by a humeral dark spot horizontally oval, and an elongated caudal peduncle dark spot, continued to the tip of the mid caudal rays, two dark brown vertical bars on the humeral region, and a single black lateral stripe with silvered border. Forty-two to forty-six scales on lateral line; fourteen to sixteen vertical scales rows. Twenty-seven to thirty-one anal fin rays. Dentary large teeth quincuspid, inner row of pre-maxilar teeth quincuspid or hexacuspid, and one maxillary tooth. Comments on the characters which permit to recognize the new species from others with the same color pattern are made.
Astyanax argyrimarginatus sp. n.; Characidae; Tetragonopterinae; taxonomy; description; Araguaia River; Brazil
