Diagnosis: Petrocephalus similis is distinguished from all other Petrocephalus species of Central Africa by the following combination of characteristics: body coloration gold/silver-white without melanin mark; 22–24 branched rays in the dorsal fin and 28-29 branched rays in the anal fin (Ref. 87308). Petrocephalus similis can be further distinguished from P. simus by its mouth opening under the posterior half of the eye vs. the anterior half of the eye in P. simus); its relatively deep caudal peduncle, the ratio caudal peduncle length and caudal peduncle depth between 2.2 and 2.6 vs. 2.8 in P. simus; and its greater number of scale rows between the anterior base of the anal fin and the lateral line, 16 or 17 vs. 12 in P. simus (Ref. 87308). Petrocephalus similis can be further distinguished from P. sullivani, by its proportionally larger mouth, the ratio head length and mouth width between 3.9 and 4.5 vs. 4.8 in P. sullivani; wider head, the ratio head length and head width between 1.6 and 1.9 vs. 2.0 in P. sullivani; and its shorter caudal peduncle, the ratio standard length and caudal peduncle length between 6.3 and 7.5 vs. 6.3 in P. sullivani (Ref. 87308). From the six Nilo-Sudanian species occurring in the Niger and Chad basins bordering the upper Sanaga, P. similis is easily distinguished from Petrocephalus pallidomaculatus, Petrocephalus soudanensis and Petrocephalus ansorgii by its complete absence of large melanin sub-dorsal spot; from Petrocephalus bane and Petrocephalus sauvagii, by its smaller mouth and fewer branched rays in the dorsal and anal fins; and from Petrocephalus bovei, by its fewer branched rays in the dorsal and anal fins and its larger maximum standard length (Ref. 87308).

Description: Body ovoid, 2.9-3.3 times longer than high, and laterally compressed (Ref. 87308). Head length 3.8-4.2 times in standard length (Ref. 87308). Mouth small, the ratio head length and mouth width between 3.9 and 4.5, subterminal, opening under the posterior half of the eye (Ref. 87308). Teeth small and bicuspid, 8-11 in a single row in the upper jaw, 19-23 in a single row in the lower jaw (Ref. 87308). Caudal peduncle thin, the ratio caudal peduncle length and caudal peduncle depth between 2.2 and 2.6 (Ref. 87308). 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 22-24 branched rays; anal fin with 28-29 branched rays (Ref. 87308). Scales cover the body, except for the head; lateral line visible and complete with 36-40 pored scales along its length; 15-17 scales between the anterior base of the anal fin and the lateral line; 12 scales around caudal peduncle (Ref. 87308). Skin on head thick, turning opaque with formalin fixation; knollenorgans visible, clustered into three distinct rosettes (Ref. 87308).

Colouration: Body background colour gold/silver-white with metallic reflection on the head, darker dorsally from midline; no large melanin spot or mark on side below dorsal fin and at the base of the pectoral fins and the caudal fin; all fins whitish-yellowish, mostly translucent (Ref. 87308).