Diagnosis: Petrocephalus frieli is distinguished from all other Petrocephalus species of Central Africa by the following combination of characteristics: body coloration brownish without an intense subdorsal pigmentation mark; 21-24 branched rays in the dorsal fin and 28-30 branched rays in the anal fin; more than 13 scale rows between the anterior base of the anal fin and the lateral line; and three distinct electroreceptive rosettes on the head (Ref. 91772).

Description: Body ovoid, longer than high, the ratio standard length and height between 2.7 and 3.2, and laterally compressed (Ref. 91772). Head length 3.4 to 3.8 times in standard length (Ref. 91772). Snout short, the ratio head length and snout length between 5.3 and 6.7, and round (Ref. 91772). Mouth small, the ratio head length and mouth width between 3.7 and 4.9, sub-terminal, opening under the posterior half of the eye (Ref. 91772). Teeth small and bicuspid; 9-14 in a single row in the upper jaw, 18-22 in a single row in the lower jaw (Ref. 91772). Caudal peduncle thin, the ratio caudal peduncle length and caudal peduncle depth between 2.3 and 2.9 (Ref. 91772). Dorsal and anal fins originate in the posterior half of the body; pre-dorsal distance slightly greater than the pre-anal distance; dorsal fin with 21-24 branched rays; anal fin with 28-30 branched rays (Ref. 91772). Scales cover the body, except for the head; lateral line visible and complete, with 37-40 pored scales along its length; 15-17 scales between the anterior base of the anal fin and the lateral line; 12 scales around the caudal peduncle (Ref. 91772). Skin on head thick, turning opaque with formalin fixation; knollenorgans visible, clustered into three distinct rosettes (Ref. 91772).

Colouration: Body background colour silver-white with metallic reflection on the flanks, darker on the snout, forehead and along the dorsal edge until the caudal peduncle; presence of a very faded subdorsal spot below the anterior part of the dorsal fin only on the specimens from Lake Bangweulu, not observed in other specimens; scattered melanophores over the body, denser and larger on the head; all fins whitish, translucent, with melanophores marking the lepidotrichia (Ref. 91772).