Diagnosis: Aphyosemion mengilai can be distinguished from all other congeners apart from A. grelli by having grey to black margins in the unpaired fins in males and females. It is distinguished from A. grelli by having more dorsal and anal fin rays in males and females, 13-14 vs. 10-12 and 15-16 vs. 13-14; by greater dorsal fin base length, 14.1-19.6% of standard length vs. 12.5-13.9%; shorter preanal length in females, 59.1-62.4% of standard length vs. 62.8-63.9%; relatively greater anal fin base length in males, 18.8-22.6% of standard length vs. 17.2-19.8%; by the more anterior insertion of dorsal fin relative to anal fin origin, ratio D/A 4-6 vs. 6-7; by having more scales on transverse series, 11-12 vs. 9-10; more scales around caudal peduncle, 15-18 vs. 12; and by the presence of light blue submarginal stripes in dorsal, anal , and caudal fin in females (Ref. 95601).

Description: Body slightly laterally compressed; dorsal profile slightly convex, maximum body depth at pelvic-fin origin; ventral profile slightly convex; profile of the caudal peduncle slightly concave to nearly straight (Ref. 95601). Snout slightly rounded, mouth directed upwards, lower jaw longer than upper jaw (Ref. 95601). Premaxilla and dentary with unicuspid, slightly curved teeth; vomerine teeth absent, anterior border of vomer thickened; post-temporal lateral process absent; anterodorsal process of the urohyal bifid (Ref. 95601). Caudal fin subtruncated; dorsal and anal fins located posterior to mid-body, tips rounded; dorsal-fin rays 13-14, first dorsal-fin ray on vertical between 4th to 6th anal-fin ray; anal-fin rays 15-16; caudal-fin rays 26-29; pectoral-fin rays 17-18; pelvic-fin rays 6 (Ref. 95601). Scales cycloid, body entirely scaled; frontal squamation G-type; scales on mid-longitudinal series 30-32+4; scales on transverse series 11-12; scales around caudal peduncle 15-16 (Ref. 95601). Frontal neuromast series 'open' type; anterior supraorbital series with three neuromasts; preopercular canal with six pores; one neuromast on each scale of median longitudinal series (Ref. 95601).

Colouration: Males with body and head metallic light blue, darker dorsally, four interrupted horizontal series of red dots on flanks, dots in close proximity from each other; two narrow oblique red bars on opercle; dorsal, anal, and caudal fin light blue, with narrow black distal margin, horizontal series of dots on the proximal parts of dorsal and anal fin; pelvic fin light blue; pectoral fin hyaline with ventral light blue margin (Ref. 95601). Females with body and head brownish-grey, two interrupted horizontal series of small red dots on flanks; pectoral fin hyaline, dark yellow at base; pelvic fin dark yellow with light blue margin; dorsal and anal fins dark yellow with narrow grey distal margins and light blue submarginal stripe; caudal fin hyaline, dark yellow at base, marginel region grey, submarginal portion light blue (Ref. 95601).