Description: Lethrinops atrilabris is a small laterally compressed, maximum body depth 2.5-2.7 times maximum head width, cichlid fish with a short, rounded snout, 27-32% of head length, small mouth low down on the head and very large eyes, 38-41% of head length (Ref. 127517). It is relatively deep-bodied, laterally compressed, deepest part of body generally well behind first dorsal fin spine; anterior upper lateral profile convex and gently curving, without a sharp inflection in curve above the eye; lower anterior lateral profile also gently curving, so that tip of snout lies well above insertion of pelvic fins (Ref. 127517). Mouth relatively small, low on head, slightly upwardly angled, snout well below horizontal plane from bottom of eye; eye extremely large, circular, generally appearing more or less touching anterior upper lateral head profile; lachrymal much wider than deep, five openings (Ref. 127517). Flank scales weakly ctenoid, cteni becoming reduced dorsally, particularly anteriorly above upper lateral line, where they transition into a cycloid state; scales on chest are relatively large, gradually transitioning in size from larger flank scales, as is typical in non-mbuna Malawian endemic haplochromines; a few small scales scattered on the proximal part of caudal fin; longitudinal line scales 31-34, cheek scales 2 (Ref. 127517). Cephalic lateral line pores inconspicuous, flank lateral line shows the usual cichlid pattern of separate upper and lower portions (Ref. 127517). Pectoral fin very long when intact, extending well past first anal spine; pelvic fins extend past vent in all specimens and past first anal spine in some; this may be a sexually dimorphic trait, with female haplochromines often having shorter pelvic fins; tips of dorsal and anal fins also prolonged, extending well past the plane through base of caudal fin in some specimens, again probably a sexually dimorphic trait, exaggerated in males; tailfin crescentic; dorsal fin with 15-16 spines and 9-10 soft rays, anal fin with 3 spines and 7-9 soft rays (Ref. 127517). Lower jaw relatively small, with thin mandibular bones, but not flattened as in some Placidochromis; jaw teeth small, short and erect; outer series in both upper and lower jaw largely unequally bicuspid, becoming more equally bicuspid posteriorly, notably in upper jaw; a single inner series of very small tricuspid teeth (Ref. 127517). Lower pharyngeal bone small, lightly built, Y-shaped, and carries small, short, laterally compressed slightly hooked, blunt, simple teeth; middle-lying 5-6 teeth on each side of posterior row slightly larger than others, but molarization lacking; about 12 teeth in midline row and about 20 on each side on posterior row; gill rakers simple, erect, fairly long and well spaced, with few, if any, reduced to small stubs near anterior part of arch; upper gill rakers 4-5, lower gill rakers 13-14 (Ref. 127517).

Colouration: Colouration of females and immatures is unknown, but, from experience of other species from this habitat, can be expected to be countershaded, sandy-coloured dorsally, with silvery flanks and probably faint vertical flank bars (Ref. 127517). Colouration of males in breeding dress: strong dark brownish vertical flank bars on silvery-white background; six bars under dorsal fin, two more on caudal peduncle and one to two on nape; head dark brown on upper surface, but paler laterally, sometimes with a dark lachrymal mark running from eye towards the mouth; eye golden brown, darker along the axis of lachrymal stripe; lips, lower jaw, throat and chest are black; dorsal fin dark golden-brown, with a series of irregular white spots or oblique stripes angled forwards from base, with broad black margin and broader white submarginal band; pectoral fins translucent, but brownish-tinted; pelvic fins black, fading to dark grey on posterior rays; anal fin black, fading to dark grey basally and marked with irregular yellowish spots and stripes; caudal fin with dark grey to black upper and lower margins, but otherwise dark golden-brown with three thin irregular vertical white bands (Ref. 127517).