Astyanax douradilho is distinguished from all congeners from the rio Uruguay basin, laguna dos Patos and rio Tramandaí systems by having 3-5 maxillary tricuspid teeth ( except A. henseli, A. laticeps, and A. paris), and by the absence of a conspicuous dark stripe from humeral region to caudal peduncle. It differs from A. henseli by having 11-12 gill rakers on lower branch of first arch (vs. 15-19), from A. laticeps by having two vertically elongated humeral spots (vs. one oval horizontally elongate), and from A. paris by having 37-39 perforated scales along the lateral line (vs. 34-36). It differs from all congeners by the following combination of characters: presence of two vertically elongate humeral spots, a conspicuous caudal spot; absence of a conspicuous dark stripe from humeral region to caudal peduncle; 3-5 maxillary tricuspid teeth; 22-24 branched anal-fin rays; 37-39 perforated scales along the lateral line; head length (26.0-29.9% SL); upper jaw length (43.8-50.6% HL); and snout length (23.0-28.6% HL) (Ref. 96006).

Description: Anal-fin rays iii, or iv, 22, 23, or 24 (Ref. 96006).