Diagnosis: Nannopetersius mutambuei is distinguished from its two congeners by the following combination of characters: lateral line with 28-30 scales and 27-30 pored scales, vs. 30-35 and 32-35 in N. ansorgii and 28-33 and 27-33 in N. lamberti; 5.5-6.5 scales between the origin of the dorsal fin and the lateral line, vs. 6.5 in N. ansorgii and 6.5-7.5 in N. lamberti; 10-13 gill rakers on the ceratobranchial of the first branchial arch, vs. 14-17 in N. ansorgii and 10-12 in N. lamberti; length of caudal peduncle 135.7-151.9% of its depth, vs. caudal peduncle nearly as long as deep in N. ansorgii and N. lamberti; snout length 26.6-29.2% of head length, vs. 18.4-23.0% in N. lamberti; eye diameter 31.7-39.6% of head length, vs. 46.0-56.2% in N. lamberti; and no broad dark band in life, however present in preserved specimens, and no additional black spots on caudal and distal blackish band on anal fin, vs. a faint dark medio-lateral band, in preserved specimens, hardly visible anteriorly but better marked on the caudal fin and no additional black spots on caudal and anal fin in N. ansorgii, and a broad, black, medio-lateral band in life as well as preserved specimens, distal part of both upper and lower caudal-fin lobe with a black spot and distal margin of anal fin blackish in N. lamberti (Ref. 79819).
Description: Small-sized species; snout somewhat shorter than eye; mouth terminal with a slightly prominent lower jaw (Ref. 79819). Gill rakers on first gill arch thin and relatively well-spaced (Ref. 79819). Teeth of outer row of upper jaw alternating with those of inner row; teeth on outer row of upper jaw tricuspid, teeth of inner row a little larger and with five cusps; usually 4 teeth on outer and 8 teeth on inner row of upper jaw; external mandibular teeth with 5 cusps; both inner teeth of lower jaw small and conical (Ref. 79819). Complete lateral line with 28-30 scales and 27-30 pored scales (Ref. 79819). Dorsal fin with 2 simple and 8 branched rays; pectoral fin longer than pelvic fin but not reaching base of latter; pelvic fin originating approximately below third branched ray of dorsal fin; caudal fin forked with sharp-pointed lobes (Ref. 79819).
Colouration: In life, general colour silvery with greenish dorsal region (Ref. 79819). Caudal and adipose fin bright red; anal, pectoral, pelvic and dorsal-fin base yellowish; distal edge of fins reddish (Ref. 79819). Peripheral parts of iris reddish; blackish opercular spot; no dark mid-lateral band visible in living specimens (Ref. 79819). In preserved specimens, general colour brownish on dorsal midline and dorsal flank above black mid-lateral band, whitish on ventral midline and ventral flank up to black mid-lateral band; black mid-lateral band extending from posterior margin of operculum to caudal-fin base up to posterior edge of median caudal-fin rays; humeral spot sometimes visible but often hidden by dark mid-lateral band (Ref. 79819).