Trichomycterus trefauti differs from all other known members of the sub-family Trichomycterinae by the autapomorphic presence of ann elliptical, vertically elongated, brown spot, at caudal-fin base (vs. without elliptical spot at caudal-fin base), and the combination of homogeneously gray color pattern (vs. yellowfish, presence of stripes or or bands, or lack of color pattern), first pectoral-fin ray prolonged as a filament (vs. not prolonged), subterminal mouth (vs. inferior or terminal mouth), two supraorbital pores at interorbital space (vs. one supraorbital pore at mesial line), caudal fin truncate with attenuated edges (vs. caudal fin rounded), pelvic fins covering anus and urogenital openings (vs.not covering), interorbital space very wide 39.8-45.9% HL(vs. more or less than 39.8-45.9% HL), maxillary barbels very long 84.2-93.0% HL (vs.more or less than 84.2-93.0% HL), rictal barbels very long 67.6-74.3% HL (vs.more or less than 67.6-74.3%HL).