From Lavoué (2012):
Body ovoid longer than high (mean SL/H = 2.9) and laterally compressed. Head length 3.6 times (mean) in standard length. Snout short (mean HL/SNL = 6.0) and round. Mouth small (mean HL/MW = 4.2), sub-terminal, opening under the posterior half of the eye. Teeth small and bicuspid, a median of 11 teeth in a single row in the upper jaw, a median of 20 teeth in a single row in the lower jaw. Dorsal and anal fins originate in the posterior half of the body (mean SL/PDD = 1.6 and mean SL/PAD = 1.7). Pre- dorsal distance slightly greater than the pre-anal distance (1.0 ≤ PDD/PAD ≤ 1.1). Median dorsal fin branched rays of 23. Median anal fin branched rays of 29. Scales cover the body, except for the head. Lateral line visible and complete, with a median of 38 pored scales along its length. Twelve scales around the caudal peduncle. Median of 16 scales between the anterior base of the anal fin and the lateral line. Caudal peduncle thin (mean CPL/CPD = 2.7). Skin on head thick, turning opaque with formalin fixation. Knollenorgans visible, clustered into the three distinct rosettes of Harder (1968).
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 the other specimens. Scattered melanophores over the body, denser and larger on the head. All fins whitish, translucent, with melanophores marking the lepidotrichia.
From Lavoué (2012):
Petrocephalus frieli sp. nov. is distinguished from all other Petrocephalus spp. of Central Africa (i.e. Lower Guinea and Congo provinces) by the following combination of characteristics: body coloration brownish without an intense subdorsal pigmentation mark; 21 to 24 branched rays in the dorsal fin (mean = 23) and 28–30 branched rays in the anal fin (mean = 29); more than 13 scale rows between the anterior base of the anal fin and the lateral line; three distinct electroreceptive rosettes on the head.
Petrocephalus frieli sp. nov. differs from P. balayi, P. binotatus, P. christyi, P. mbossou, P. odzalaensis, P. pulsivertens and P. zakoni by the absence of any distinct black marking, including the absence of a distinct subdorsal spot.
Maximum SL observed = 90.7 mm (holotype).