Diagnosis: Melanorivulus imperatrizensis is distinguished from all remaining congeners by the following character states in combination: in males, all fins orange (vs. orange fins absent in other species of Melanorivulus, except for M. dapazi, which possess a orange distal stripe on all fins), and caudal-fin basis orange (vs. never this color pattern in the remaining Melanorivulus); small and few reddish-brown spots in caudal fin (vs. reddishbrown spots absent in caudal fin, or red spots present in M. parnaibensis), red spots not arranged as chevrons (vs. red spots arranged as chevrons or similar to the letter Y), small and few oblique red spots only on the posterior portion of body (vs. many red spots spread over the body or forming oblique red bars), black pigmentation absent on the head (vs. presence of intense black pigmentation on the head either as lines, bars or spots , with the exception of M. illuminatus and M. pinima), broad dark orange stripe on the ventral margin of the caudal-fin (vs. ventral margin of the caudal-fin dark gray or black), anal-fin rounded (vs. pointed, except M. illuminatus), dorsal-fin origin at vertical through the base of the 9th anal-fin ray, between neural spines of vertebrae 22-23 (vs. 18-21 in M. crixas, M. faucireticulatus, M. giarettai, M. javahe, M. kayabi, M. leali, M. megaroni, M. pinima, M. pindorama, M. punctatus, M. planaltinus, M. rossoi, M. rubromarginatus, M. ubirajarai, and M. zygonectes), anal-fin origin between pleural ribs of vertebrae 17-18 (vs. 14-16 in M. javahe, M. kayapo, M. pinima, M. planaltinus, 13-16 in M. scalaris); lower caudal-fin length 23.9-29.3% in SL (vs. 29.4-32, 1 % SL in M. planaltinus, 31.6-42.3% SL in remaining congeners); females of M. imperatrizensis can be distinguished from all other species of the genus by the caudal-fin with 2 large reddish-brown spots vertically arranged, the 1st on the upper half of caudal-fin basis, the 2nd on the lower half of caudal-fin basis (vs. only one black spot on upper base of caudal-fin or 2 black spots in M. cyanopterus, sometimes in M. punctatus 2 smaller spots), 3-4 reddish-brown bars in the caudal-fin (vs. 3 black bars in M. violaceus, 6 faint gray narrow bars in M. faucireticulatus, 6-7 black bars in M. paresi, 3-5 dark gray bars in M. parnaibensis), lower caudal-fin length 25.2-25.3% in SL (vs. 26.0-27.9% SL in M. decoratus 28.7-41.8% SL in remaining congeners).