Melanorivulus nigromarginatus is similar to M. egens and M. linearis, and differs from all other members of the M. pictus group by having a black distal marginal stripe on the anal fin in males (vs. absence). It can be diagnosed from M. egens and M. linearis by having the following characters: melanophores strongly concentrated on the post-orbital and humeral regions, forming a stripe (vs. weakly concentrated, not forming distinct marks); red dots on the anteroventral portion of flank present (vs. absent); red bars on most portion of caudal fin present in males (vs. bars absent in M. egens and bars restricted to the dorsal portion of the caudal fin in M. linearis); and red chevron-shaped marks irregularly distributed on the flank (vs. regularly). In addition, but not diagnostic to differentiate all speci¬mens, M. nigromarginatus usually possesses 31-33 scales in the longitudinal series (vs. 29-31 in M. egens and M. linearis) (Ref. 116860).