Diagnosis: Petrocephalus mbossou is distinguished from all other Petrocephalus species in Central Africa by the following combination of characteristics: dorsal fin with 22-24 branched rays; anal fin with 26-28 branched rays; mouth inferior and small, with ration head length and mouth width 4.2-5.1, with 9-11 teeth in a single row in the upper jaw and 14-22 teeth in a single row in the lower jaw; distance from the anterior extremity of the snout to the corner of the mouth only 2.8-3.4 times in head length (Ref. 85331, 113655). Weak pigmentation pattern with the presence of two black markings on each side of the body: (1) an irregular patch below the anterior base of the dorsal fin (first to sixth/seventh rays); (2) an irregularly-shaped mark centered at the base of the caudal fin that does not extend onto the rayed portions of the upper and lower caudal fin lobes (Ref. 85331).

Description: Body ovoid, longer than high, the ration standard length and height 2.6-3.0, and laterally compressed (Ref. 85331, 113655). Head length 3.3-3.6 times in standard length (Ref. 113655). Eye large, the ratio head length and eye-diameter 3.3-3.8 (Ref. 85331, 113655). Snout short, the ratio head length and snout length 5.2-7.2, and round (Ref. 85331, 113655). Mouth small, the ratio head length and mouth width 4.2-5.1, opening ventrally under the posterior half of the eye (Ref. 85331, 113655). Teeth small and bicuspid, 9-11 in a single row in the upper jaw, 14-22 in a single row in the lower jaw (Ref. 85331, 113655). Caudal peduncle relatively thin, the ratio caudal peduncle length and caudal peduncle depth 2.3-2.9 (Ref. 85331, 113655). Both the dorsal and anal fins originate in the posterior half of the body; the pre-dorsal distance is slightly greater than the pre-anal distance; dorsal fin with 22-24 branched rays; anal fin with 26-28 branched rays (Ref. 85331, 113655). Scales cover the body, except for the head; lateral line visible and complete with 35-37 pored scales along its length; 12 circumpeduncular scales (Ref. 85331, 113655). Skin on head thick, turning opaque with formalin fixation, and containing numerous knollenorgan electroreceptors organized into only two rosettes (Ref. 85331).

Colouration: Body mostly white-silver; dorsum slightly darker than the rest of the body; pigmentation pattern consists of two distinct black markings: a black patch of irregular shape below the anterior base of the dorsal fin, and an irregularly-shaped mark centered at the base of the caudal fin that does not extend onto the rayed portions of the upper and lower caudal fin lobes; the fins themselves are translucent (Ref. 85331).