Diagnosis: A large bodied tilapia, with a relatively large and forward-facing snout, body at least 2.8 times as long as deep (Ref. 4967, 118638). The species can be distinguished by following characters: teeth very small, in narrow bands; pharyngeal bone narrow, its width about 26% length of head, with fine teeth, crowded posteriorly, the blade equal to the median length of the toothed area in young, about 1.5 times as long in adults; pectoral fin usually less than 36% of standard length; anal spines III-IV; scales of chest and belly very small, in some individuals showing an abrupt transition from the flank scales for a short distance behind the pectoral-pelvic interspace; in some the dorsal scales may also be small; and caudal peduncle at least 1.3 times longer than deep (Ref. 2, 4967). Males characterised by an intricate melanin pattern on the head, flank scales that are darker towards posterior side, and a background light grey-blue colour; the dorsal region is darker than the ventral, and ripe males with dark pectoral, anal, caudal fins, while the dorsal fin is dark with orange margins; females with less contrasting colours, with flanks uniformly grey or olive, with a lighter ventral region (Ref. 118638).

Description: The upper profile of the head is approximately straight, the mouth terminal with the jaws meeting evenly (Ref. 2). Mouth rather large, extending to between nostril and eye (Ref. 1870, 2989). The teeth are smaller and more slender than even in Oreochromis jipe; in the largest specimens a few of the outer teeth are tricuspid and some of the posterior may be unicuspid, but most are bicuspid; in adults many teeth are worn or broken, probably the result of browsing on the rocks; the inner series are irregular and the toot-bands are narrower than in O. jipe and O. pangani; in the biggest fish teeth of the inmost row are unicuspid (Ref. 2). The pharyngeal teeth are small and slender, restricted to a small area (Ref. 2). Scales cycloid; the number of scales in the lateral series shows equal modes at 34 and 35, higher numbers being often due to interpretation of smaller scales, in some cases on one side only; 18-19 scales on upper lateral line, 15-16 scales on lower lateral line; 3 series of scales on the cheek; scales on cheek rather small (Ref. 2, 1870, 2989, 55061). Dorsal fin with XVI-XVII spines and 11-13 soft rays, last dorsal spine longest; the total anal rays are 14-16, with III-IV spines and 10-13 soft rays; pectoral fin pointed, a little shorter than head, not extending to origin of anal fin; pelvic fin reaching vent; caudal fin truncate (Ref. 2, 1870, 55061). Vertebrae 31-34 (Ref. 2).

Colouration: Adult metallic blue-black on upper parts of head and body, shading to iridescent dark greenish blue on lower parts of gill-cover and jaws and to blackish bronze on the flanks; breast speckled dark green and white, belly grey on white; some individuals with edge of dorsal fin dark orange (Ref. 2). Large males are a deep royal blue and every scale is distinct; this colour shades down to a pearly white on the belly, with a tinge of yellow on the throat; the fins are also royal blue and all points are a flaming orange or a bright vermilion colour (Ref. 36900). The females are grass-green with dark green backs (Ref. 36900). Young bronze on flanks, greenish bronze on back, cream on belly; head as adult but not so dark; caudal and dorsal fins dark green, lower fins paler iridescent green (Ref. 2). A checkered effect in both young and adult produced by the darker anterior part of each scale (Ref. 2). In alcohol, young fishes have a well-marked dark mid-lateral longitudinal band, continuous or emphasized on 11-13 narrow dark vertical bars, which also bear a series of 5-7 blotches above the upper lateral line; the tilapia-mark, present through variably emphasized at these sizes, is horizontally elongate (Ref. 2).