Diagnosis: Petrocephalus leo is distinguished from all other Petrocephalus species of central Africa by the following combination of characteristics: absence of electroreceptive rosettes on head, body colouration brownish with distinct black mark at the base of pectoral fin, but no subdorsal ovoid black mark; 21 to 24 branched dorsal-fin rays and 26 to 29 branched anal-fin rays (Ref. 113655). Absence of the three typical Petrocephalus electroreceptive rosettes on the head distinguish Petrocephalus leo from most of its congeners, except P. microphthalmus, P. haullevillii, P. schoutedeni and P. zakoni; but it can be easily distinguished from these species by its unique pattern of melanin markings: a distinct black spot at the origin of the pectoral fins but no subdorsal pigmentation mark (Ref. 113655). Petrocephalus leo can be further distinguished from P. zakoni by its slender body, body depth 2.9-3.4 times in standard length vs. 2.5-2.7 times in P. zakoni; its relatively shorter dorsal fin, dorsal fin length 4.4-5.0 times in standard length vs. 4.0-4.3 times; and its smaller eye, eye diameter 3.7-4.4 times in head length vs. 3.1-3.3 times (Ref. 113655). It can be further distinguished from P. microphthalmus, P. haullevillii and P. schoutedeni, by a higher dorsal-fin ray count, 21-24 vs. less than 20 in the other species (Ref. 113655).

Description: Petrocephalus leo is a small-sized species within the genus; body ovoid, 2.9-3.4 times longer than high and laterally compressed; head length between 3.1-3.7 times in standard length (Ref. 113655). Snout short, 5.2-7.4 times in head length, and round; mouth small, 3.8-4.9 times in head length, sub-terminal, opening under posterior half of eye; teeth small and bicuspid, 7-9 in a single row in the upper jaw, 15-23 in a single row in lower jaw (Ref. 113655). Dorsal and anal fins originate in posterior half of body; pre-dorsal distance slightly greater than pre-anal distance; dorsal fin with 21-21 branched rays; anal fin with 26-29 branched rays (Ref. 113655). Body scaled except for head; lateral line visible and complete with 34-38 pored scales along its length; 12 scales around caudal peduncle; 9-12 scales between anterior base of anal fin and lateral line (Ref. 113655). Caudal peduncle thin, its depth 1.9-2.5 times in its length (Ref. 113655). Skin on head thick, turning opaque with formalin fixation; knollenorgans visible, but not clustered into the three distinct rosettes (Ref. 113655).

Colouration: In life: body background brown/copper with metallic reflection on flanks, darker dorsally with belly whitish/silver with many dense melanophores visible; caudal peduncle darker and lower margin of dorsal and anal fins blackish; a distinct black mark at base of the pectoral fins and a crescent-shaped mark at base of the caudal fin extending onto upper and lower lobes of caudal fin; all fins mostly whitish-brownish and translucent, anterior of dorsal fin darker; eye black (Ref. 113655).