Moenkhausia mutum can be differentiated from other species of Moenkhausia, except M. hemigrammoides, M. nigromarginata and Moenkhausia rubra by having intense diffuse dark pigmentation on the dorsal and anal fin-rays, especially in anteriormost rays and in the distal portion (vs. fins hyaliane or only with scattered chromatophores). It differs from M. rubra by having 20-23, modally 21 branched anal-fin rays (vs. 17-20, modally 19) and by having a vertically elongate humeral spot (vs. approximately rounded); from M. hemigrammoides by having intense dark pigmentation on pelvic fin (vs. hyaline), by the anal fin with a relative diffuse dark pigmentation (vs. presence of a black oblique line extending from the urogenital cavity to the tip of second or third branched anal-fin ray), and by the absence of a well-defined black spot on the distal portion of the dorsal fin (vs. presence); and from M. nigromarginata by lacking a series of longitudinal dark zigzag stripes on body formed by the concentration of dark chromatophores in the upper and lower portions of the exposed area of scales (vs. presence) and by having a vertically elongate roughly rectangular humeral spot surpassing the lateral line ventrally (vs. humeral spot approximately rounded, with an anteroventral projection not surpassing the lateral line ventrally) (Ref. 109750).